Anatomy & Physiology II · In-depth topic guides

Innate Immunity

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This topic covers the body's built-in, non-specific defense systems — the innate immune system. It explores the physical and chemical barriers that block pathogen entry, the cellular defenders (phagocytes and natural killer cells) that eliminate invaders, the molecular cascades (complement and interferon) that amplify the response, and the coordinated inflammatory and febrile reactions that contain and destroy threats. Understanding innate immunity is foundational to immunology and explains why simple barrier breaches (like a cut) can trigger a full-body response.

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10.1 Innate vs. Adaptive Immunity: The Big Picture

The immune system operates on two tiers. The innate immune system responds immediately (minutes to hours) with the same generic defenses regardless of the pathogen. The adaptive immune system takes days to mobilize but delivers a highly specific, memory-equipped response. Together they form a layered defense.

Feature	Innate Immunity	Adaptive Immunity
Response Time	Immediate (minutes–hours)	Delayed (days to weeks)
Specificity	Broad pattern recognition (PAMPs)	Highly specific (individual antigens)
Memory	None — same response every time	Long-lasting immunological memory
Key Cells	Neutrophils, macrophages, NK cells, dendritic cells	T lymphocytes, B lymphocytes, plasma cells
Key Molecules	Complement, interferon, lysozyme, defensins	Antibodies (immunoglobulins), TCRs, cytokines
Receptors	Pattern recognition receptors (TLRs, NLRs) — germline-encoded	Antigen receptors (TCR, BCR) — somatically rearranged
Self/Non-Self	Recognizes absent "self" markers and generic pathogen patterns	Highly discriminating; requires self-tolerance training
Response to Repeat Exposure	Identical	Faster and stronger (anamnestic response)

The innate system does more than hold the line — it activates and instructs the adaptive response through antigen presentation and cytokine signaling.

10.2 First Line of Defense: Physical and Chemical Barriers

These barriers prevent pathogens from ever entering the body's internal environment.

Physical Barriers

Skin (epidermis): The stratum corneum is a dense layer of dead, keratinized epithelial cells that pathogens cannot penetrate. Continuous shedding physically removes attached microbes. The dryness and slightly acidic pH (≈5.0) of skin further inhibit microbial growth.
Mucous Membranes: Line all body cavities open to the exterior (respiratory, GI, urogenital tracts). Goblet cells secrete mucus, a sticky glycoprotein-rich fluid that traps microbes and debris.
Cilia: Hair-like projections on respiratory epithelial cells beat in coordinated waves, propelling the mucociliary escalator — a mucus "conveyor belt" that sweeps trapped particles toward the pharynx for swallowing or expulsion.
Fluid Flow: Mechanical flushing by tears (lacrimal apparatus), urine, saliva, and vaginal secretions continuously washes microbes away from epithelial surfaces.

Chemical Barriers

Lysozyme: An enzyme found in tears, saliva, mucus, and breast milk. It cleaves the peptidoglycan cell wall of bacteria, causing lysis — especially effective against Gram-positive bacteria.
Sebum: Oily secretion from sebaceous glands that forms a protective acidic film (pH 3–5) on skin. Contains fatty acids that inhibit bacterial and fungal growth.
Gastric Acid: Hydrochloric acid (HCl) in the stomach maintains a pH of ~1.5–3.5, killing most ingested microorganisms.
Defensins: Small, cationic antimicrobial peptides produced by epithelial cells and neutrophils. They insert into microbial membranes, forming pores that disrupt membrane integrity and kill the microbe.
Antimicrobial Peptides (AMPs): Beyond defensins, cells produce cathelicidins, histatins (saliva), and dermcidin (sweat) — each targeting different microbial vulnerabilities.
Lactoferrin: Binds free iron, an essential nutrient for bacterial growth, effectively starving pathogens.
Normal Microbiota: Resident bacteria (commensals) competitively exclude pathogens by occupying attachment sites and consuming available nutrients.

10.3 Second Line of Defense: Phagocytes

When barriers are breached, phagocytes — cells that engulf and digest foreign material — form the second line of defense.

Neutrophils

Most abundant leukocyte (50–70% of circulating WBCs)
First responders to infection: arrive within minutes
Short-lived (hours to days); die after one round of phagocytosis
Primary target: bacteria and fungi
Contain granules loaded with lysozyme, defensins, and reactive oxygen species (ROS)

Macrophages

Tissue-resident phagocytes derived from circulating monocytes
Long-lived; can phagocytose repeatedly
Functions: phagocytosis, antigen presentation (display pathogen fragments on MHC II molecules to activate T cells), secretion of cytokines that orchestrate inflammation, and cleanup of cellular debris
Named by tissue: Kupffer cells (liver), alveolar macrophages (lungs), microglia (CNS), osteoclasts (bone)

Dendritic Cells

Bridge between innate and adaptive immunity
Master antigen-presenting cells (APCs): capture antigen at infection site, migrate to lymph nodes, and present to naïve T cells
Found in skin (Langerhans cells), mucosal surfaces, and lymphoid organs

Mechanism of Phagocytosis

Phagocytosis proceeds through a six-step sequence:

Chemotaxis: Phagocytes detect chemical gradients of microbial products, complement fragments (C5a), and cytokines, and migrate toward the highest concentration.
Adhesion: Phagocyte surface receptors bind to PAMPs (pathogen-associated molecular patterns) on the microbe. Opsonization — coating the microbe with complement protein C3b or antibodies — dramatically enhances this step.
Engulfment (Pseudopod Formation): The phagocyte extends cytoplasmic extensions (pseudopods) that wrap around the microbe.
Phagosome Formation: The pseudopods fuse, internalizing the microbe in a membrane-bound vesicle called a phagosome.
Phagolysosome Formation: The phagosome fuses with lysosomes (organelles containing digestive enzymes), forming a phagolysosome.
Digestion and Destruction: The microbe is destroyed by:
- Lysosomal enzymes (lysozyme, proteases, lipases, nucleases)
- Respiratory burst: A sharp increase in oxygen consumption by the phagocyte, producing reactive oxygen species (superoxide anion O₂⁻, hydrogen peroxide H₂O₂, hypochlorous acid HOCl via myeloperoxidase) that are potently microbicidal
- Nitric oxide and other reactive nitrogen intermediates

Summary Table: Major Phagocytic Cells

Cell Type	Location	Lifespan	Antigen Presentation	Primary Targets
Neutrophil	Blood (circulating), tissue (migrating)	Hours–days	No	Bacteria, fungi
Macrophage	Tissues (resident)	Weeks–months	Yes (MHC II)	Bacteria, debris, tumor cells
Dendritic Cell	Skin, mucosa, lymph nodes	Days–weeks	Yes (MHC II — most potent APC)	Any antigens; bridge to adaptive

10.4 Natural Killer (NK) Cells

Natural killer cells are large granular lymphocytes of the innate system that do not require prior exposure to a pathogen to act.

Recognition Strategy — "Missing Self": NK cells survey the body for cells that fail to display normal levels of MHC class I molecules. All healthy nucleated cells express MHC I on their surface — it acts as a "self" identification badge. Virally infected cells and tumor cells often downregulate MHC I to evade cytotoxic T cells. NK cells detect this absence and attack. This is called missing-self recognition.

Killing Mechanism:

NK cell binds to the target cell.
Releases cytotoxic granules containing:
- Perforin: Forms pores in the target cell membrane
- Granzymes: Serine proteases that enter through perforin pores and trigger apoptosis (programmed cell death) via caspase activation
Also secretes IFN-γ, which activates macrophages and shapes the adaptive response.

Regulation: NK activity is controlled by a balance of activating and inhibitory receptors. Killer inhibitory receptors (KIRs) bind MHC I and deliver inhibitory signals — when MHC I is present, the NK cell is restrained. When MHC I is absent or reduced, inhibition is lifted and killing proceeds.

10.5 The Complement System

The complement system is a cascade of approximately 30 plasma proteins (synthesized mainly by the liver) that circulate in inactive forms and become sequentially activated. The system amplifies both innate and adaptive immunity.

Three Activation Pathways

All three pathways converge on the activation of C3, the central protein of the complement system.

Classical Pathway: Activated when C1q binds to antibody-antigen complexes (IgG or IgM bound to pathogen surface). This links complement to the adaptive immune response. Also activated by C-reactive protein and some microbial surfaces directly.
Lectin Pathway: Activated when mannose-binding lectin (MBL) binds to mannose residues on microbial surfaces. Mannose is a sugar found on many pathogens but not on healthy human cells. MBL-associated serine proteases (MASPs) then cleave C4 and C2.
Alternative Pathway: Activated spontaneously by the slow hydrolysis of C3 in plasma (C3 tickover). The hydrolyzed C3 (C3(H₂O)) binds factor B, which is cleaved by factor D to form an initial C3 convertase. This pathway is amplified on microbial surfaces (which lack regulatory proteins that human cells possess) and serves as an amplification loop for the other pathways.

The Central Step — C3 Convertase: All three pathways generate a C3 convertase (C4b2a for classical/lectin; C3bBb for alternative) that cleaves C3 into:

C3a: Small fragment — an anaphylatoxin that promotes inflammation
C3b: Large fragment — covalently binds to the pathogen surface (opsonization) and also combines with existing C3 convertase to form C5 convertase

Three Major Outcomes

Outcome	Mediators	Mechanism	Biological Effect
Opsonization	C3b (and its cleavage product iC3b)	Coats pathogen surface; recognized by complement receptor CR1 on phagocytes	Dramatically enhances phagocytosis ("buttering the toast for eating")
Inflammation	C3a, C5a (anaphylatoxins)	Bind receptors on mast cells and basophils → degranulation (histamine release); C5a is also a potent chemotactic factor for neutrophils	Vasodilation, increased vascular permeability, smooth muscle contraction, recruitment of phagocytes
Cytolysis (MAC)	C5b, C6, C7, C8, multiple C9	Assemble into the Membrane Attack Complex (MAC), a pore that spans the microbial membrane	Osmotic lysis of Gram-negative bacteria and enveloped viruses; water and ions flood in through the pore

Regulation: Host cells are protected by complement regulatory proteins (DAF/CD55, MCP/CD46, CD59/protectin, Factor H, Factor I) that inactivate complement components on self-surfaces. Pathogens lack these regulators, so complement activation proceeds unchecked on their surfaces.

10.6 Interferon

Interferons (IFNs) are cytokine proteins that "interfere" with viral replication. They are produced by infected cells as an early warning system.

Type I Interferons (IFN-α and IFN-β): Produced by virtually all virus-infected cells (especially plasmacytoid dendritic cells for IFN-α, fibroblasts and epithelial cells for IFN-β). They act in a paracrine and autocrine fashion:
- Bind to IFN receptors on neighboring uninfected cells, triggering these cells to produce antiviral proteins (such as PKR — protein kinase R, which blocks viral protein synthesis, and 2′-5′ oligoadenylate synthetase, which activates RNase L to degrade viral RNA)
- This induces an antiviral state — the cell is primed to resist viral replication but is not harmed
- Enhance NK cell cytotoxic activity
- Upregulate MHC I expression, making infected cells more visible to cytotoxic T cells

Type II Interferon (IFN-γ): Produced primarily by activated NK cells and T cells (Th1). Functions:
- Potent macrophage activator — enhances phagocytosis, respiratory burst, and antigen presentation
- Upregulates MHC I and MHC II expression
- Promotes differentiation of Th1 cells (adaptive immunity)
- Bridges innate and adaptive responses

Clinical Use: Recombinant IFN-α is used therapeutically for hepatitis B, hepatitis C, and certain cancers (e.g., melanoma, hairy cell leukemia).

10.7 Inflammation

Inflammation is the body's localized, non-specific response to tissue injury, infection, or irritation. The suffix "-itis" denotes inflammation of a specific tissue (e.g., appendicitis, dermatitis, hepatitis).

Cardinal Signs

The five classic signs (described by Celsus and Galen):

Rubor — Redness (vasodilation → increased blood flow)
Calor — Heat (increased blood flow from deeper, warmer tissues)
Tumor — Swelling (increased vascular permeability → fluid and proteins leak into tissue — edema)
Dolor — Pain (chemical mediators like bradykinin and prostaglandins sensitize nerve endings; physical pressure from edema also contributes)
Functio laesa — Loss of function (combined effects of swelling and pain)

Vascular Changes (Early Phase)

Brief vasoconstriction (seconds) at the site of injury — a rapid, transient neural reflex
Vasodilation of arterioles and capillaries: induced by histamine (from mast cells and basophils), nitric oxide, and prostaglandins. Results in increased blood flow (causing redness and heat) and slower flow (allowing cellular events)
Increased vascular permeability: Endothelial cells of post-capillary venules contract (mediated by histamine, bradykinin, leukotrienes), creating gaps. Plasma proteins (complement, antibodies, clotting factors) and fluid leak into the interstitial space, forming an exudate

Cellular Events (Late Phase)

Margination: Slowed blood flow allows leukocytes (especially neutrophils) to move to the periphery of the vessel, close to the endothelial wall.
Rolling: Leukocytes transiently bind to selectins (P-selectin, E-selectin) expressed on activated endothelial cells. This low-affinity interaction creates a rolling motion along the vessel wall.
Firm Adhesion: Inflammatory cytokines (TNF-α, IL-1) upregulate integrins (LFA-1, Mac-1) on leukocytes and ICAM-1/VCAM-1 on endothelial cells. Integrin-ICAM binding arrests the leukocyte firmly on the endothelium.
Diapedesis (Transmigration): Leukocytes squeeze between endothelial cells through intercellular junctions, exiting the bloodstream into the tissue.
Chemotaxis: Once in the tissue, leukocytes follow chemical gradients of chemokines, complement fragments (C5a), bacterial products, and leukotrienes to the site of injury.

Sequence of Cellular Arrival:

Neutrophils arrive first (within 1–2 hours); dominate the first 24 hours; form pus (dead neutrophils + debris + bacteria)
Monocytes/macrophages arrive after 24–48 hours; dominate chronic inflammation; clean up debris and initiate tissue repair

Chemical Mediators of Inflammation

Mediator	Primary Source	Key Actions
Histamine	Mast cells, basophils, platelets	Vasodilation, increased permeability (immediate)
Prostaglandins (PGE₂, PGI₂)	Many cells (COX pathway from arachidonic acid)	Vasodilation, pain sensitization, fever
Leukotrienes (LTB₄, LTC₄, LTD₄)	Leukocytes, mast cells (LOX pathway from arachidonic acid)	Chemotaxis (LTB₄), increased permeability, bronchoconstriction
Bradykinin	Kinin system (plasma)	Vasodilation, increased permeability, pain
C3a, C5a	Complement cascade	Mast cell degranulation (→ histamine), C5a chemotaxis
TNF-α, IL-1	Macrophages, other cells	Endothelial activation (↑ adhesion molecules), fever, acute-phase protein induction
Nitric Oxide (NO)	Endothelial cells, macrophages	Vasodilation, microbicidal (macrophages)
Platelet-Activating Factor (PAF)	Various cells	Platelet activation, increased permeability, bronchoconstriction

10.8 Fever

Fever (pyrexia) is a systemic elevation of body temperature, orchestrated by the hypothalamus.

Mechanism:

Exogenous pyrogens (microbial products like LPS — lipopolysaccharide from Gram-negative bacteria) stimulate macrophages and other cells.
These cells release endogenous pyrogens — primarily IL-1, IL-6, and TNF-α.
Endogenous pyrogens travel via the bloodstream to the hypothalamus (the body's thermostat).
They stimulate the anterior hypothalamus to produce prostaglandin E₂ (PGE₂) via the cyclooxygenase-2 (COX-2) pathway.
PGE₂ raises the hypothalamic set point — the brain now perceives normal body temperature as "too cold."
The body responds with heat-generating and heat-conserving mechanisms: shivering (muscle activity), vasoconstriction (reduces heat loss), piloerection, and behavioral changes (seeking warmth). This phase produces the chill sensation.
When the set point returns to normal (crisis or lysis), vasodilation and sweating dissipate heat — the flush phase.

Benefits of Fever:

Inhibits pathogen growth: Many bacteria require iron and specific temperatures; elevated temperature reduces iron availability and slows replication
Enhances immune function: Accelerates phagocytosis, T-cell proliferation, and interferon activity
Increases metabolic rate: Heats up the "engine" of the immune response
Note: Excessive fever (>40.5 °C / 105 °F) can denature proteins and is dangerous, but moderate fever is a protective response, not a malfunction

Antipyretics (aspirin, ibuprofen, acetaminophen) work by inhibiting COX enzymes, reducing PGE₂ production, and thus lowering the set point back toward normal.

10.9 Clinical Correlations

Leukocytosis: An abnormally elevated white blood cell count (>11,000/µL), commonly seen in acute bacterial infections. Neutrophils are preferentially elevated (neutrophilia). The bone marrow releases stored and immature neutrophils (band forms — a "left shift") into circulation.
Leukopenia: An abnormally low WBC count (<4,000/µL), seen in some viral infections, sepsis, bone marrow failure, or chemotherapy. Reduced immune capacity increases infection risk.
Chronic Granulomatous Disease (CGD): An inherited immunodeficiency caused by a defective NADPH oxidase enzyme. Phagocytes can ingest microbes but cannot mount a respiratory burst — they cannot produce the reactive oxygen species needed to kill catalase-positive organisms (e.g., Staphylococcus aureus, Aspergillus). Patients suffer from recurrent, severe bacterial and fungal infections. Diagnosis: negative nitroblue tetrazolium (NBT) test.
Complement Deficiencies:
- C3 deficiency: Profoundly increased susceptibility to pyogenic (pus-forming) bacterial infections, since all three pathways converge on C3. Recurrent Neisseria and encapsulated bacterial infections.
- C5–C9 (MAC) deficiency: Specifically increased susceptibility to Neisseria species (N. meningitidis, N. gonorrhoeae) — MAC-mediated lysis is critical for killing these organisms.
- C1 esterase inhibitor deficiency (Hereditary Angioedema): Uncontrolled classical pathway activation → excessive bradykinin production → recurrent episodes of non-pitting, non-pruritic angioedema (swelling of face, airway, GI tract).
Leukocyte Adhesion Deficiency (LAD): Defect in integrin CD18 → neutrophils cannot firmly adhere to endothelium → cannot undergo diapedesis. Patients present with high circulating neutrophil counts but no pus formation at infection sites.

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ELI-10: Physical and Chemical Barriers — Your Body's Castle

Imagine your body is a castle. The skin is like a thick outer wall made of tough, dead bricks (keratinized cells) that enemies can't dig through. The sticky mucus in your nose and throat is like a moat that traps invaders. Tiny hairs called cilia are like little brooms sweeping the trapped enemies up and out. Your tears, saliva, and stomach acid are like chemical guards that dissolve enemies on contact. Together, these walls and traps stop most germs before they ever get inside.

ELI-10: Phagocytes — The Pac-Man Cells

When an enemy does break through the wall, special cells called phagocytes rush to the scene. Neutrophils are like the fast, first-responding security team — they arrive quickly, eat one enemy, and die. Macrophages are like veteran guards stationed permanently in each neighborhood — they eat many enemies over and over again and can even show enemy "wanted posters" (antigen presentation) to call in backup. The eating process works like Pac-Man: the cell smells the enemy (chemotaxis), wraps its arms around it (pseudopods), and digests it in an internal "stomach" (phagolysosome) using powerful acid and bleach-like chemicals (respiratory burst).

ELI-10: Natural Killer Cells — The ID Checkers

Natural killer cells patrol the body like security guards checking everyone's ID badge. Every healthy cell carries an ID badge called MHC I. When a virus infects a cell or a cell turns cancerous, it often hides its badge so other immune cells can't see it. But NK cells are smart — they look for cells without badges. No badge? That cell gets eliminated. They inject the suspicious cell with proteins (perforin and granzymes) that tell it to self-destruct (apoptosis), like a guard pressing a button that makes a dangerous robot self-destruct.

ELI-10: Complement — The Domino Cascade

The complement system is like a set of dominoes placed around a battlefield. There are three ways to tip the first domino (classical, lectin, or alternative pathway), but once it falls, all roads lead to the same middle domino — C3. When C3 breaks apart, three things happen: (1) pieces coat the enemy like sticky jam, making them easier for Pac-Man cells to eat (opsonization), (2) other pieces act like alarm bells calling more defenders to the area (inflammation), and (3) some pieces drill holes directly into the enemy's shell, making it burst like a water balloon (MAC). The best part? The dominoes only keep falling on enemy surfaces — your own cells have shields that stop the cascade.

ELI-10: Interferon — The Fire Alarm

When a cell gets infected by a virus, it screams "FIRE!" by releasing interferon proteins. These proteins fly to all the neighboring cells and tell them: "Barricade your doors — a virus is coming!" Each neighbor cell then builds antiviral defenses, preparing to fight if the virus arrives. Interferon is like a fire alarm system: the burning building (infected cell) warns all nearby buildings to turn on their sprinklers (antiviral proteins) so the fire (virus) can't spread to them.

ELI-10: Inflammation — The Fire Department Response

If a splinter pokes through your castle wall, your body sends a fire department. First, blood vessels widen (vasodilation) to bring more help — that's the redness and heat. Then the vessel walls get leaky so fluid and soldiers can get out — that's the swelling. The leaking fluid carries proteins that fight the enemy, but the pressure and chemical signals hurt — that's the pain. The first soldiers (neutrophils) arrive fast like firefighters with hoses. Later, cleanup crews (macrophages) come to fix the damage. The whole response — redness, heat, swelling, pain, and not being able to use the body part — is what we call inflammation. It's not a mistake; it's the body working exactly as designed.

ELI-10: Fever — Turning Up the Thermostat

Your brain has a thermostat (the hypothalamus) that keeps your body at about 37°C (98.6°F). When germs invade, they trigger chemicals (pyrogens) that tell the thermostat to raise the target temperature — like someone turning up the heat on a cold day. Your body then shivers to generate heat and closes off blood vessels near the skin to keep the heat in, making you feel cold even though you're getting hotter. Why turn up the heat? Because many germs grow poorly at higher temperatures and your immune cells work better when it's warmer. Once the germs are defeated, the thermostat resets, you sweat to cool down, and your temperature returns to normal.

Check yourself

12 review questions from the chapter. Try each one, then open the answer.

Which of the following features is characteristic of the innate immune system but NOT the adaptive immune system?
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Immunological memory that strengthens with repeated exposure B. Highly specific recognition of individual antigens via rearranged receptors C. Immediate response within minutes to hours using pre-formed, germline-encoded receptors D. Response that takes days to weeks to fully mobilize during a first exposure Answer: C. Immediate response within minutes to hours using pre-formed, germline-encoded receptors Why It's the Answer: The innate immune system uses germline-encoded pattern recognition receptors (TLRs, NLRs) that are ready to go immediately — no rearrangement or clonal expansion is required. Option A is a hallmark of adaptive immunity (memory B and T cells). Option B describes the antigen-specific receptors (TCR, BCR) unique to adaptive immunity. Option D describes the lag time of the primary adaptive response, not innate immunity. ELI-10: Innate immunity is like a security guard who uses the same metal detector on everyone — instant and always ready. Adaptive immunity is like a detective who builds a specific case file on each criminal, which takes time but makes future captures much faster.
A neutrophil encounters a bacterium coated with C3b at the site of a wound. Which of the following correctly orders the steps of phagocytosis that follow?
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Chemotaxis → Pseudopod formation → Adhesion → Phagolysosome → Phagosome → Digestion B. Adhesion → Chemotaxis → Pseudopod formation → Phagosome → Phagolysosome → Digestion C. Chemotaxis → Adhesion → Pseudopod formation → Phagosome → Phagolysosome → Digestion D. Adhesion → Pseudopod formation → Phagosome → Chemotaxis → Phagolysosome → Digestion Answer: C. Chemotaxis → Adhesion → Pseudopod formation → Phagosome → Phagolysosome → Digestion Why It's the Answer: The neutrophil first detects the chemical gradient and moves toward the bacterium (chemotaxis). It then binds the C3b-coated microbe (adhesion/opsonization-enhanced binding). It extends pseudopods to wrap around the target (engulfment). The engulfed microbe is internalized in a phagosome, which then fuses with lysosomes to form a phagolysosome, where digestive enzymes and ROS destroy the microbe. Option A places adhesion after pseudopod formation — the cell must bind before it can engulf. Option B places chemotaxis before adhesion incorrectly in sequence (this one has chemotaxis second, which is wrong too, but primarily adhesion can't precede the movement toward the target). Option D places chemotaxis after phagosome formation, which is logically impossible — the cell must first move to the target. ELI-10: Think of phagocytosis like eating a cookie: first you smell it and walk toward it (chemotaxis), then you grab it (adhesion), wrap your fingers around it (pseudopods), put it in your mouth (phagosome), mix it with saliva (phagolysosome), and digest it in your stomach (digestion). The steps must go in this order — you can't swallow before grabbing!
All of the following are chemical barriers of the innate immune system EXCEPT:
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Lysozyme in tears and saliva B. Hydrochloric acid in the stomach C. Ciliated epithelial cells in the respiratory tract D. Defensins produced by epithelial cells and neutrophils Answer: C. Ciliated epithelial cells in the respiratory tract Why It's the Answer: Ciliated epithelial cells are a physical barrier — they mechanically sweep mucus and trapped microbes out of the airways via the mucociliary escalator. They do not secrete a chemical that directly kills or inhibits microbes. Lysozyme (A) enzymatically destroys bacterial cell walls. Gastric HCl (B) denatures proteins and kills microbes through extreme acidity. Defensins (D) are antimicrobial peptides that insert into and disrupt microbial membranes. ELI-10: Chemical barriers are like sprays and poisons that destroy enemies. Physical barriers are like walls and brooms that block or sweep enemies away. Cilia are tiny brooms — they sweep, but they don't spray any chemicals.
A 2-year-old boy is brought to the pediatrician with a history of recurrent abscesses and pneumonia caused by Staphylococcus aureus and Aspergillus species. Laboratory testing reveals his neutrophils can ingest bacteria normally, but a nitroblue tetrazolium (NBT) test is negative. Which component of the phagocytic killing mechanism is most likely defective?
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Chemotaxis receptors B. Complement receptor CR1 (C3b receptor) C. NADPH oxidase enzyme D. Lysosomal proteases Answer: C. NADPH oxidase enzyme Why It's the Answer: The NBT test assesses the respiratory burst — the ability of phagocytes to produce reactive oxygen species. A negative NBT test with normal ingestion points to chronic granulomatous disease (CGD), caused by a defect in the NADPH oxidase enzyme complex, which is required for the respiratory burst that produces superoxide. Option A (chemotaxis defect) would prevent migration to the site, not ingestion. Option B (CR1 defect) would impair opsonization-enhanced binding, reducing ingestion — but ingestion is stated as normal. Option D (lysosomal proteases) could impair killing by a different mechanism but would not cause a negative NBT test specifically. ELI-10: Neutrophils kill bacteria with a "bleach spray" (respiratory burst). In CGD, the spray bottle is broken — the cell can grab and swallow the germ (ingestion is normal), but it can't spray the bleach that actually kills it. The NBT test turns blue when the spray works; no color change means the spray bottle (NADPH oxidase) is broken.
A natural killer cell encounters two cells: Cell X expresses normal MHC I levels, while Cell Y has markedly reduced MHC I expression. What happens?
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The NK cell kills both Cell X and Cell Y via perforin and granzyme release B. The NK cell kills Cell X but spares Cell Y C. The NK cell spares Cell X but kills Cell Y D. The NK cell ignores both cells because NK cells only target antibody-coated cells Answer: C. The NK cell spares Cell X but kills Cell Y Why It's the Answer: NK cells operate on the "missing self" principle. Killer inhibitory receptors (KIRs) on NK cells bind MHC I and deliver inhibitory signals. Cell X with normal MHC I activates these inhibitory signals, restraining the NK cell. Cell Y with low MHC I fails to engage the inhibitory receptors, so inhibition is lifted and the NK cell proceeds to kill via perforin/granzyme-induced apoptosis. Option A is wrong because normal MHC I actively inhibits NK killing. Option B reverses the correct outcome. Option D is incorrect — NK cells are part of innate immunity and act without antibodies (that's antibody-dependent cellular cytotoxicity by other cells). ELI-10: NK cells are like security guards checking badges. If every cell must show a badge (MHC I), the guard stops anyone without one. Cell X shows its badge — "you may pass." Cell Y has hidden its badge (common in viral infections and cancer) — the guard says "no badge, you're coming with me."
A researcher studying the complement system observes that C3 is cleaved into C3a and C3b in a mouse model lacking all antibodies. Which complement activation pathway(s) could still generate this C3 cleavage?
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Classical pathway only B. Lectin and alternative pathways C. Classical and lectin pathways D. All three pathways can still function Answer: B. Lectin and alternative pathways Why It's the Answer: The classical pathway requires antibody (IgG or IgM) bound to antigen for C1q to initiate the cascade. In an antibody-deficient model, the classical pathway cannot be activated. The lectin pathway uses mannose-binding lectin (MBL) that directly binds microbial sugars — no antibodies needed. The alternative pathway activates via spontaneous C3 tickover (hydrolysis) — also antibody-independent. Option A is wrong because the classical pathway needs antibodies. Option C includes the classical pathway, which requires antibodies. Option D is wrong because the classical pathway is inoperative. ELI-10: Think of the three complement pathways as three different keys to the same lock (C3). The classical pathway key only works when antibodies are around. The lectin and alternative pathway keys work on their own just by recognizing common germ patterns. Without antibodies, you've lost one key but the other two still open the lock.
A cell infected with an RNA virus releases interferon-α. Which of the following best describes the effect of IFN-α on neighboring uninfected cells?
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It directly destroys viral particles in the extracellular fluid B. It induces an antiviral state by triggering production of proteins like PKR and 2′-5′ oligoadenylate synthetase C. It opsonizes viral particles to enhance phagocytosis D. It forms pores in the membranes of nearby infected cells, causing their lysis Answer: B. It induces an antiviral state by triggering production of proteins like PKR and 2′-5′ oligoadenylate synthetase Why It's the Answer: IFN-α binds to receptors on neighboring cells and induces the synthesis of antiviral proteins. PKR blocks viral protein synthesis (by phosphorylating eIF-2), and 2′-5′ oligoadenylate synthetase activates RNase L, which degrades viral RNA. Together these create an antiviral state. Option A is incorrect — interferon does not directly destroy free viral particles. Option C misattributes opsonization (C3b, antibodies) to interferon. Option D describes the MAC of complement or perforin from NK cells, not interferon. ELI-10: Interferon is a text message, not a weapon. The infected cell texts "VIRUS ATTACK" (IFN-α) to all its neighbors. Each neighbor reads the text, runs to the hardware store, and buys door locks and security cameras (PKR and RNase L). Now they're prepared if the virus tries to break in. Interferon doesn't fight the virus itself — it tells others to get ready.
A 25-year-old woman steps on a rusty nail. Within minutes, the wound site becomes red, warm, swollen, and painful. An antihistamine partially reduces the redness and swelling but does not eliminate them completely. Which of the following mediators is LEAST likely to be affected by the antihistamine?
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Histamine released from mast cells B. Prostaglandins produced via the COX pathway C. Mast cell degranulation triggered by the injury D. Histamine binding to H1 receptors on endothelial cells Answer: B. Prostaglandins produced via the COX pathway Why It's the Answer: Antihistamines block histamine receptors (H1) and thus prevent histamine-mediated vasodilation and increased permeability. However, inflammation involves many mediators. Prostaglandins are produced via the COX pathway, independently of histamine. They cause vasodilation and pain sensitization and are unaffected by antihistamines — which is why NSAIDs (COX inhibitors) are often used alongside antihistamines for inflammation. Options A, C, and D are all histamine-related: antihistamines block H1 receptors (D), which prevents histamine's effects regardless of how much histamine is released (A) or degranulation (C). ELI-10: Inflammation is like a team of workers all contributing to a construction project. Histamine is one worker, prostaglandins are another. Antihistamines only stop the histamine worker — the prostaglandin worker keeps hammering away. That's why the swelling goes down a bit but doesn't fully go away. You'd need an NSAID (like ibuprofen) to stop the prostaglandin worker too.
The membrane attack complex (MAC) is assembled from which complement components and produces what outcome?
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C3a and C5a — triggers mast cell degranulation B. C3b only — coats the pathogen surface to enhance phagocytosis C. C5b, C6, C7, C8, and multiple C9 molecules — forms a transmembrane pore causing osmotic lysis D. C1q, C1r, C1s — binds to antibody-antigen complexes Answer: C. C5b, C6, C7, C8, and multiple C9 molecules — forms a transmembrane pore causing osmotic lysis Why It's the Answer: After C5 is cleaved by C5 convertase, C5b initiates the terminal pathway. C5b binds C6 and C7, the complex inserts into the lipid bilayer, C8 joins, and then multiple C9 molecules polymerize to form a cylindrical pore — the MAC. Water and ions rush in, and the microbe lyses. Option A describes anaphylatoxins (inflammation), not lysis. Option B describes opsonization alone, not MAC. Option D describes the initiating complex of the classical pathway, not the terminal lytic complex. ELI-10: C3a and C5a are alarm bells. C3b is a sticky flag. But C5b through C9 are actual weapons — they assemble like a tube-shaped drill bit that punctures a hole in the germ's outer shell. The germ fills with water and pops like a water balloon. This is the MAC — the "drill team" of the complement system.
Which of the following correctly traces the sequence of events that produces fever during a bacterial infection?
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Hypothalamus releases IL-1 → IL-1 acts on macrophages to release LPS → LPS raises the hypothalamic set point B. Bacterial LPS (exogenous pyrogen) stimulates macrophages → macrophages release IL-1 and TNF-α (endogenous pyrogens) → hypothalamic PGE₂ synthesis → set point raised C. Macrophages release histamine → histamine acts on the hypothalamus → vasodilation raises core temperature D. Complement C3a stimulates the hypothalamus directly → set point raised → shivering produces heat Answer: B. Bacterial LPS (exogenous pyrogen) stimulates macrophages → macrophages release IL-1 and TNF-α (endogenous pyrogens) → hypothalamic PGE₂ synthesis → set point raised Why It's the Answer: The correct chain is: exogenous pyrogen (LPS from Gram-negative bacteria) → macrophages release endogenous pyrogens (IL-1, IL-6, TNF-α) → these act on the hypothalamus to induce COX-2 → PGE₂ synthesis → set point elevation. Option A reverses the sequence — LPS is exogenous (from bacteria), and IL-1 is endogenous (from host cells). Option C incorrectly has histamine from macrophages affecting the hypothalamus; histamine primarily drives local vascular changes, and macrophages release cytokines, not histamine as their primary product. Option D is wrong because C3a is an anaphylatoxin that acts locally on mast cells and smooth muscle, not directly on the hypothalamus to raise the set point. ELI-10: Bacteria have a molecule on their surface called LPS — think of it as a "germ fingerprint." When macrophages touch this fingerprint, they release signaling molecules (IL-1, TNF-α) that travel to the brain's thermostat. The thermostat then produces PGE₂, which is like turning the dial from 37°C to 39°C. Your body suddenly thinks it's too cold and starts shivering to warm up — even though you're actually getting a fever.
A patient with a genetic deficiency in C3 would be expected to have an increased susceptibility to bacterial infections. Which of the following complement functions would NOT be impaired in this patient?
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Opsonization of bacteria by C3b B. Mast cell degranulation mediated by C3a C. Formation of the membrane attack complex D. Mannose-binding lectin binding to microbial surfaces Answer: D. Mannose-binding lectin binding to microbial surfaces Why It's the Answer: Mannose-binding lectin (MBL) is a pattern recognition molecule of the lectin pathway. It binds mannose on microbial surfaces independently of C3 — this is an upstream recognition event that occurs before C3 is recruited. MBL binding itself is not impaired by C3 deficiency; however, everything downstream (C4 and C2 cleavage, C3 cleavage, and terminal pathway) would be impaired. Options A, B, and C all require C3 or its cleavage products: C3b is needed for opsonization, C3a for mast cell activation, and C3b forms part of the C5 convertase needed to generate C5b for MAC assembly. ELI-10: C3 is like the middle domino in a long chain. If C3 is missing, everything after it stops — no sticky tags (C3b), no alarm bells (C3a), no drill team (MAC). But MBL is like the finger that tips the very first domino — the finger itself still works fine in a C3-deficient person. It just can't push anything forward because the domino chain is broken at C3.
A biopsy of an infected wound taken 6 hours after injury is most likely to show a predominance of which cell type?
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Macrophages B. Lymphocytes C. Neutrophils D. Eosinophils Answer: C. Neutrophils Why It's the Answer: Neutrophils are the first responders of the cellular immune response. They arrive within 1–2 hours and dominate the first 24 hours of acute inflammation. They are the most abundant leukocyte in circulation and respond rapidly to chemotactic signals (C5a, LTB₄, IL-8). Macrophages (A) arrive later, peaking at 24–48 hours. Lymphocytes (B) are primarily adaptive immune cells and are more characteristic of chronic inflammation. Eosinophils (D) are associated with parasitic infections and allergic reactions, not typical acute bacterial wound infections. ELI-10: Think of neutrophils as the local fire department — they're stationed nearby and rush to the fire within minutes. Macrophages are like the specialized disaster response team — they arrive the next day after the initial crisis is under control. At 6 hours, you'd mostly see firefighters (neutrophils), not the cleanup crew (macrophages).