Immune System

Innate Immunity

Innate immunity provides the first line of defense—rapid (minutes to hours), nonspecific, and lacking immunological memory. Physical and chemical barriers: skin (keratinized epithelium, acidic pH, antimicrobial peptides), mucous membranes (mucus traps pathogens, cilia sweep them out), lysozyme in tears and saliva (degrades bacterial peptidoglycan), stomach acid (pH ~2), and commensal microbiota that compete with pathogens. Cellular components: phagocytes (neutrophils—first responders, most abundant WBC, short-lived; macrophages—tissue-resident, long-lived, also serve as antigen-presenting cells; dendritic cells—professional APCs that bridge innate and adaptive immunity), natural killer (NK) cells (kill virus-infected and tumor cells by detecting reduced MHC I expression—the 'missing self' hypothesis), mast cells and basophils (release histamine, mediate allergic responses), eosinophils (target parasites). Soluble factors: complement system (~30 plasma proteins, activated by classical [antibody-bound], lectin [mannose on pathogens], or alternative [spontaneous C3 hydrolysis] pathways—all converge on C3 convertase, leading to opsonization [C3b], inflammation [C3a, C5a], and membrane attack complex [C5b-C9, forms pores]). Inflammation: vasodilation (histamine, prostaglandins → redness, heat), increased vascular permeability (edema, swelling), recruitment of phagocytes (chemotaxis), and systemic effects (fever via pyrogens → prostaglandin E2 → hypothalamic set-point increase).

Adaptive Immunity

Adaptive immunity is specific, slower to develop (days on first exposure), and generates immunological memory for rapid response upon re-exposure. Two branches: humoral immunity (B lymphocytes → antibodies, targets extracellular pathogens and toxins) and cell-mediated immunity (T lymphocytes, targets intracellular pathogens and abnormal cells). Lymphocytes develop in primary lymphoid organs: B cells mature in bone marrow; T cells mature in the thymus (where positive selection ensures MHC restriction and negative selection eliminates self-reactive clones—central tolerance). Mature but naive lymphocytes circulate through secondary lymphoid organs (lymph nodes, spleen, mucosal-associated lymphoid tissue/MALT) where they encounter antigens. Each naive lymphocyte bears a unique receptor (BCR for B cells, TCR for T cells) generated by V(D)J recombination during development—this generates the enormous diversity (~10^11 possible specificities) needed to recognize any potential pathogen. The adaptive response has four phases: (1) antigen recognition by naive lymphocytes, (2) clonal expansion (proliferation of antigen-specific clones), (3) differentiation into effector cells (plasma cells, cytotoxic T cells, helper T cells) and memory cells, and (4) effector function to eliminate the pathogen. The primary response is slower and produces mainly IgM; the secondary response (upon re-exposure) is faster, stronger, and produces mainly IgG—the basis of vaccination.

Antigens and Antibodies

An antigen is any molecule recognized by an antibody or TCR. Epitopes (antigenic determinants) are the specific regions of an antigen bound by the receptor. Antibodies (immunoglobulins) are Y-shaped proteins composed of two identical heavy chains and two identical light chains held together by disulfide bonds. Each chain has a variable (V) region at the N-terminus (antigen-binding site, generated by V(D)J recombination) and a constant (C) region that determines effector function. Five isotypes determined by heavy-chain constant region: IgG (most abundant in serum, crosses placenta, opsonization, neutralization, activates complement), IgM (pentamer, first antibody produced in primary response, excellent complement activator), IgA (dimer, secreted across mucosal surfaces—tears, saliva, breast milk, intestinal secretions), IgE (binds mast cells and basophils, mediates allergic responses and defense against parasites), IgD (membrane-bound on naive B cells, functions as BCR alongside IgM). Antibody effector functions: neutralization (blocking pathogen binding to host cells), opsonization (coating pathogen to enhance phagocytosis via Fc receptors on phagocytes), complement activation (classical pathway via C1q binding to antibody-antigen complexes), antibody-dependent cell-mediated cytotoxicity/ADCC (NK cells kill antibody-coated target cells).

B Cells and T Cells

B cells mediate humoral immunity. Each naive B cell expresses membrane-bound IgM and IgD as its B-cell receptor (BCR). When BCR binds cognate antigen, the B cell internalizes, processes, and presents antigen peptides on MHC class II. Helper T cells (Th) that recognize the same antigen provide co-stimulatory signals (CD40L-CD40 interaction, cytokines) that drive B-cell activation, proliferation, and differentiation. Activated B cells undergo: (1) class switching (isotype switching—from IgM to IgG, IgA, or IgE, mediated by AID enzyme and cytokines from Th cells: IFN-γ → IgG, IL-4 → IgE, TGF-β → IgA), (2) somatic hypermutation (point mutations in V regions of proliferating B cells, selecting for higher affinity—affinity maturation), and (3) differentiation into either plasma cells (antibody factories, short-lived unless in bone marrow niches) or memory B cells (long-lived, ready for rapid secondary response). T cells come in two major subsets distinguished by co-receptors. CD4+ T cells (helper T cells) recognize antigens on MHC class II and differentiate into subsets: Th1 (IFN-γ, activates macrophages, cell-mediated immunity), Th2 (IL-4, IL-5, IL-13, humoral immunity, anti-parasite), Th17 (IL-17, mucosal defense, autoimmunity), Treg (regulatory T cells, express FoxP3, suppress immune responses, prevent autoimmunity). CD8+ T cells (cytotoxic T cells) recognize antigens on MHC class I and kill infected cells via perforin (pore-forming) and granzymes (induce apoptosis) or Fas-FasL interaction (death-receptor apoptosis).

MHC Presentation

Major histocompatibility complex (MHC) molecules are cell-surface glycoproteins that display peptide fragments for T-cell recognition. MHC class I is expressed on all nucleated cells, presents endogenous peptides (derived from intracellular proteins—viral, tumor, self) to CD8+ cytotoxic T cells. Peptides are generated by the proteasome, transported into the ER by TAP (transporter associated with antigen processing), loaded onto MHC I with help from chaperones (tapasin, calreticulin), and the peptide-MHC I complex traffics to the cell surface. MHC class II is expressed primarily on professional antigen-presenting cells (dendritic cells, macrophages, B cells), presents exogenous peptides (derived from extracellular proteins taken up by phagocytosis or endocytosis) to CD4+ helper T cells. Exogenous proteins are degraded in acidified endosomes/lysosomes; the invariant chain (Ii) blocks premature peptide binding in the ER; CLIP (class II-associated invariant chain peptide) is removed by HLA-DM to allow antigenic peptide binding. Cross-presentation: some dendritic cells can present exogenous antigens on MHC class I to CD8+ T cells —important for immunity to viruses that do not infect dendritic cells. MHC genes (HLA in humans) are the most polymorphic genes in the genome—this diversity determines which peptides an individual can present and underlies transplant rejection (MHC mismatch) and disease susceptibility (certain HLA alleles increase risk for autoimmune diseases).

Clonal Selection

Clonal selection is the central principle of adaptive immunity: each lymphocyte bears a unique antigen receptor; when that receptor binds its cognate antigen, the lymphocyte is selected to proliferate, generating a clone of identical effector cells specific for that antigen. This occurs in secondary lymphoid organs. For B cells: naive B cells encounter antigen in lymph node follicles, receive T-cell help at the T-B border, proliferate in germinal centers where somatic hypermutation and class switching occur, and exit as plasma cells or memory B cells. For T cells: naive T cells enter lymph nodes via high endothelial venules (HEVs), scan dendritic cells presenting peptides on MHC; if TCR binds cognate peptide-MHC with sufficient affinity and receives co-stimulation (B7 on DC binding CD28 on T cell), the T cell is activated. Without co-stimulation, TCR binding alone induces anergy (unresponsiveness)—a peripheral tolerance mechanism. Some self-reactive lymphocytes inevitably escape central tolerance; peripheral tolerance mechanisms (anergy, deletion via apoptosis, Treg suppression, immune privilege in certain sites like the eye and brain) prevent autoimmunity. The clonal selection process explains why primary responses take ~7–10 days (rare antigen-specific clones must be selected and expanded) while secondary responses take only ~1–3 days (expanded memory populations already exist).

Vaccination and Immunological Memory

Vaccination exploits immunological memory by exposing the immune system to non-pathogenic forms of a pathogen (or its antigens), generating memory lymphocytes without causing disease. Vaccine types: live attenuated (weakened but replication-competent—MMR, varicella, yellow fever; strong immune response, contraindicated in immunocompromised), inactivated/ killed (chemically inactivated—polio/IPV, hepatitis A; safer but weaker, require boosters), subunit/recombinant (purified antigens—hepatitis B surface antigen, HPV capsid proteins), toxoid (inactivated bacterial toxins—tetanus, diphtheria), and mRNA (lipid nanoparticle-encapsulated mRNA encoding viral antigen—COVID-19 vaccines; host cells produce the antigen, which is then presented on MHC I and II). Adjuvants (e.g., aluminum salts) enhance the immune response by creating a depot effect and stimulating innate immunity. Herd immunity: when a sufficient proportion of the population is immune (through vaccination or prior infection), transmission is interrupted, protecting unvaccinated individuals. The threshold depends on pathogen R0 (basic reproduction number): herd immunity threshold = 1−1/R0. Active immunity: the individual's own immune system responds and generates memory (natural infection or vaccination). Passive immunity: preformed antibodies are transferred (maternal IgG across placenta, IgA in breast milk, antitoxin or immune globulin therapy)—immediate protection but no memory generated. The MCAT may ask you to distinguish active from passive immunity based on mechanism, duration, and source of protection.

How it works

The immune system operates as a discrimination engine: self vs. non-self. Innate immunity recognizes pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (TLRs, NLRs) and responds immediately—phagocytosis, complement activation, inflammation. If the pathogen breaches innate defenses, dendritic cells carry antigens to lymph nodes where they present peptides on MHC to naive T cells. A T cell whose TCR matches the peptide-MHC complex proliferates (clonal selection) and differentiates. CD4+ helper T cells orchestrate the response: they activate B cells (which produce antibodies), CD8+ cytotoxic T cells (which kill infected cells), and macrophages (which destroy engulfed pathogens). After pathogen clearance, most effector cells die; memory cells persist for decades, enabling rapid secondary responses. The system's Achilles' heel is autoimmunity: when self-tolerance mechanisms fail, the same destructive power targets the body's own tissues. The MCAT expects you to trace the logic: antigen encounter → presentation → T-cell recognition → effector response, and to identify where in this sequence a given immunodeficiency or therapeutic intervention acts.

How it works

The immune system operates as a discrimination engine: self vs. non-self. Innate immunity recognizes pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (TLRs, NLRs) and responds immediately—phagocytosis, complement activation, inflammation. If the pathogen breaches innate defenses, dendritic cells carry antigens to lymph nodes where they present peptides on MHC to naive T cells. A T cell whose TCR matches the peptide-MHC complex proliferates (clonal selection) and differentiates. CD4+ helper T cells orchestrate the response: they activate B cells (which produce antibodies), CD8+ cytotoxic T cells (which kill infected cells), and macrophages (which destroy engulfed pathogens). After pathogen clearance, most effector cells die; memory cells persist for decades, enabling rapid secondary responses. The system's Achilles' heel is autoimmunity: when self-tolerance mechanisms fail, the same destructive power targets the body's own tissues. The MCAT expects you to trace the logic: antigen encounter → presentation → T-cell recognition → effector response, and to identify where in this sequence a given immunodeficiency or therapeutic intervention acts.

Comparisons

• B/B (Innate vs. adaptive): Distinguish by speed, specificity, memory, and cell types involved. Innate = macrophages, neutrophils, NK, complement. Adaptive = B cells, T cells, antibodies.
• B/B (MHC restriction): CD8+ T cells recognize MHC I (all nucleated cells); CD4+ T cells recognize MHC II (professional APCs only). This is the most tested immunology fact on the MCAT.
• B/B (Antibody isotypes): IgM first, IgG most abundant and crosses placenta, IgA mucosal, IgE allergy/parasites. Know the order of appearance in primary vs. secondary response.
• B/B (Vaccination): Active immunity (own immune system, memory) vs. passive immunity (pre-formed antibodies, no memory). Herd immunity threshold.
• C/P (Antibody structure): Disulfide bonds, noncovalent interactions in antigen binding, protein biochemistry principles.
• P/S (Stress and immunity): Chronic stress suppresses immune function via cortisol; connects to psychoneuroimmunology passages.

Common confusions

• Confusing MHC class I vs. II restriction. CD8 binds MHC I (8 × 1 = 8). CD4 binds MHC II (4 × 2 = 8). Mnemonic: Rule of 8. Or: MHC I x CD8 = 8; MHC II x CD4 = 8.
• Thinking antibodies directly kill pathogens. Antibodies do NOT kill—they tag (opsonize), neutralize, and activate complement and NK cells. The killing is done by phagocytes, complement MAC, and cytotoxic T cells.
• Forgetting that the secondary immune response is faster AND stronger because memory cells already exist. Primary: lag ~7-10 days, mainly IgM. Secondary: lag ~1-3 days, mainly IgG, higher titer.
• Confusing active and passive immunity. Active = you make your own antibodies and memory cells (infection or vaccine). Passive = you receive pre-made antibodies (maternal, immune globulin)—immediate but temporary, no memory.
• Not knowing which cell type is affected in HIV/AIDS. HIV infects CD4+ T cells (helper T cells), macrophages, and dendritic cells. Loss of CD4+ T cells cripples both humoral and cell-mediated immunity.
• Thinking all immune cells are lymphocytes. Neutrophils, macrophages, dendritic cells, NK cells, mast cells, and eosinophils are NOT lymphocytes (though NK cells are lymphoid lineage). Lymphocytes = B cells + T cells + NK cells (by lineage).
• Forgetting that MHC I presents ENDOGENOUS antigens (from inside the cell—viral proteins, tumor antigens) while MHC II presents EXOGENOUS antigens (from outside, taken up by phagocytosis/endocytosis).
• Mixing up autoimmunity and immunodeficiency. Autoimmunity = immune system attacks self (overactive). Immunodeficiency = immune system cannot fight pathogens (underactive). The treatments are opposite (immunosuppression vs. immune support/replacement).

Quick review

• Innate immunity: rapid, nonspecific, no memory. Barriers, phagocytes, NK cells, complement, inflammation. Adaptive: slower, highly specific, memory. B cells (antibodies), T cells (cellular).
• MHC class I on all nucleated cells → presents endogenous peptides to CD8+ cytotoxic T cells. MHC class II on professional APCs → presents exogenous peptides to CD4+ helper T cells.
• CD8+ T cells bind MHC I (Rule of 8: 8×1=8). CD4+ T cells bind MHC II (4×2=8).
• Antibody isotypes: IgM (first, pentamer), IgG (most abundant, crosses placenta), IgA (mucosal, dimer), IgE (allergy, parasites), IgD (BCR on naive B cells).
• Primary response: slow (~7-10 days), mainly IgM. Secondary response: fast (~1-3 days), stronger, mainly IgG. Basis of vaccination.
• Active immunity: own immune system produces antibodies + memory (infection or vaccine). Passive immunity: preformed antibodies received (maternal IgG/IgA, immune globulin)—temporary, no memory.
• Clonal selection: antigen binds specific lymphocyte receptor → that lymphocyte proliferates into a clone of identical effector and memory cells.
• Complement: classical (antibody), lectin (mannose), alternative (spontaneous). All converge at C3 convertase → opsonization (C3b), inflammation (C3a, C5a), MAC lysis (C5b-C9).
• T-cell activation requires TWO signals: TCR binding MHC-peptide (signal 1) + co-stimulation B7-CD28 (signal 2). Without co-stimulation → anergy.
• HIV infects CD4+ T cells, macrophages, dendritic cells. Loss of CD4+ T cells causes immunodeficiency (AIDS).
• Autoimmunity = overactive immune response against self. Immunodeficiency = underactive, cannot fight pathogens.
• Germinal center reactions: class switching (cytokine-directed isotype change), somatic hypermutation (affinity maturation). Occur in lymph node follicles with T-cell help.