Immunology for Dental Hygiene: Innate and Adaptive Immunity

Core Review

Innate Immunity: The First Line of Defense

Innate immunity is present at birth, non-specific, does not require prior exposure, and has no memory. It responds within minutes to hours and uses germline-encoded receptors (pattern recognition receptors, PRRs) that recognize pathogen-associated molecular patterns (PAMPs).

Physical and chemical barriers (first line):

• Intact skin and mucosa (epithelial barrier)
• Saliva: Contains lysozyme (digests bacterial cell wall peptidoglycan), lactoferrin (iron-sequestering), peroxidase (generates reactive oxygen species), secretory leukocyte protease inhibitor (SLPI), histatins, and defensins
• Gingival crevicular fluid (GCF): Contains antibodies (IgG, IgA), complement proteins, and neutrophils — an inflammatory exudate present in small amounts even in health
• Mucus: Traps microorganisms
• Normal flora: Competes with pathogens for nutrients and binding sites

Cellular components of innate immunity:

• Neutrophils (PMNs): Most abundant white blood cell (50-70% of circulating WBCs). First responders to infection and inflammation. Phagocytose and kill bacteria through:
  • Phagolysosome fusion with release of lytic enzymes and reactive oxygen species (ROS — myeloperoxidase system)
  • Neutrophil extracellular traps (NETs) — DNA webs containing antimicrobial proteins
  • Primary (azurophilic) granules: myeloperoxidase, defensins, elastase
  • Secondary (specific) granules: lysozyme, lactoferrin
  • NBDHE relevance: Neutrophils are the primary defense in the gingival sulcus. Defects in neutrophil number (neutropenia) or function (chronic granulomatous disease, leukocyte adhesion deficiency) are associated with severe, early-onset periodontitis.
• Macrophages: Derived from circulating monocytes. Professional phagocytes and antigen-presenting cells (APCs). Produce pro-inflammatory cytokines (IL-1, TNF-α, IL-6) critical in periodontitis pathogenesis. Also produce anti-inflammatory cytokines (IL-10, TGF-β) during resolution.
• Dendritic cells: Professional APCs bridging innate and adaptive immunity. Langerhans cells in the oral epithelium are dendritic cells that capture antigens, migrate to lymph nodes, and present processed antigen to na•ve T cells.
• Natural Killer (NK) cells: Lymphocytes that kill virus-infected cells and tumor cells without prior sensitization. Recognize cells lacking MHC class I ("missing self"). Produce IFN-γ.
• Mast cells: Contain histamine and other vasoactive mediators; mediate allergic responses.

Soluble components of innate immunity:

• Complement system: A cascade of >30 serum proteins activated through three pathways:
  • Classical pathway: Activated by antibody-antigen complexes (IgG or IgM bound to antigen)
  • Alternative pathway: Activated directly by microbial surfaces (spontaneous C3 hydrolysis) — part of innate immunity
  • Lectin pathway: Activated by mannose-binding lectin (MBL) binding to carbohydrates on microbial surfaces
  • All three pathways converge at C3 cleavage → C3a (anaphylatoxin, promotes inflammation), C3b (opsonin, enhances phagocytosis) → C5 cleavage → C5a (potent chemotactic factor for neutrophils), C5b initiates C5b-9 membrane attack complex (MAC) → lysis of Gram-negative bacteria
  • NBDHE relevance: Complement is active in GCF; elevated complement cleavage products correlate with periodontitis severity
• Cytokines (innate): IL-1, TNF-α, IL-6 (pro-inflammatory, pyrogenic), chemokines (IL-8/CXCL8 attracts neutrophils), type I interferons (antiviral)
• Acute phase proteins: C-reactive protein (CRP — opsonin, binds phosphocholine on bacteria, activates complement), mannose-binding lectin (MBL)

Inflammation — Cardinal Signs:

1. Rubor (redness) — vasodilation
2. Calor (heat) — increased blood flow
3. Tumor (swelling) — increased vascular permeability, edema
4. Dolor (pain) — mediators (bradykinin, prostaglandins) sensitizing nociceptors
5. Functio laesa (loss of function) — added by Galen

Adaptive Immunity: Specific, Memory-Based, Amplified

Adaptive immunity requires prior exposure (or immunization) to develop, is highly specific, and features immunological memory. It is mediated by lymphocytes (B cells, T cells) and takes days to weeks to fully develop on first exposure.

Two arms of adaptive immunity:

1. Humoral immunity: B lymphocytes → plasma cells → antibodies. Targets extracellular pathogens and toxins.
2. Cell-mediated immunity (CMI): T lymphocytes. Targets intracellular pathogens (viruses, some bacteria), tumor cells, and mediates delayed-type hypersensitivity and transplant rejection.

The Five Immunoglobulins (Antibodies)

This is among the most heavily tested immunology concepts on the NBDHE:

IgG — The Workhorse:

• Most abundant immunoglobulin in serum (75-80% of total)
• Four subclasses: IgG1, IgG2, IgG3, IgG4
• THE ONLY immunoglobulin that crosses the placenta (via neonatal Fc receptor, FcRn) — provides passive immunity to the fetus and neonate
• Primary antibody of the secondary immune response
• Activates complement (classical pathway) — IgG3 > IgG1 > IgG2 (IgG4 does not)
• Opsonizes bacteria for phagocytosis
• Neutralizes toxins and viruses
• Longest half-life (~21-23 days)
• Oral relevance: IgG is the predominant immunoglobulin in GCF (derived from serum transudate). IgG levels in GCF increase in periodontitis.

IgA — The Mucosal Defender:

• Secretory IgA (sIgA): Dimeric form with J chain and secretory component (SC). The secretory component is a fragment of the polymeric immunoglobulin receptor (pIgR) added during transcytosis through epithelial cells. SC protects IgA from proteolytic degradation in the harsh mucosal environment.
• Found in secretions: saliva, tears, colostrum/breast milk, respiratory secretions, GI secretions, genitourinary secretions
• THE primary immunoglobulin in SALIVA
• Functions: Neutralizes pathogens and toxins at mucosal surfaces, prevents bacterial adhesion (immune exclusion), does NOT fix complement or opsonize
• Two subclasses: IgA1 (predominant in serum) and IgA2 (predominant in secretions, more resistant to bacterial proteases)
• Oral relevance: Salivary sIgA prevents bacterial adhesion to oral surfaces. sIgA deficiency is associated with increased respiratory infections, allergies, and possibly increased caries risk.

IgM — The First Responder:

• Pentameric structure (five monomers linked by J chain) — largest antibody, molecular weight ~970 kDa
• First antibody produced in the PRIMARY immune response (appears within 3-5 days of antigen exposure)
• Most potent complement activator (classical pathway)
• Excellent agglutinin (binds particulate antigens and clumps them)
• Does NOT cross the placenta → elevated IgM in cord blood indicates intrauterine infection
• Primary antibody response to blood group antigens (ABO) — IgM anti-A and anti-B are naturally occurring (not induced by prior exposure)
• Membrane-bound IgM (monomer) serves as the B cell antigen receptor

IgE — Allergy and Parasites:

• Lowest serum concentration (trace, ~0.0003 mg/mL)
• Binds to high-affinity FcεRI receptors on mast cells and basophils → antigen cross-linking triggers degranulation (histamine release) → Type I immediate hypersensitivity
• Primary defense against helminthic (worm/parasitic) infections
• Does NOT fix complement, does NOT opsonize
• Oral relevance: IgE-mediated reactions to dental materials (latex, rubber dam, local anesthetics — rare), oral allergy syndrome (cross-reactivity between pollen and food allergens)

IgD — The B Cell Receptor:

• Trace amounts in serum
• Membrane-bound IgD (along with membrane IgM) serves as the na•ve B cell antigen receptor
• Function of secreted IgD is poorly understood; may play a role in respiratory mucosal immunity

Primary vs. Secondary Immune Response

This is a classic NBDHE concept:

Primary response (first exposure to antigen):

• Lag phase: 5-10 days before detectable antibody
• IgM appears first (within 3-5 days), followed by IgG
• Lower total antibody titer
• Lower antibody affinity (affinity maturation has not yet occurred)
• Shorter duration
• This is why you get sick the first time you encounter a pathogen

Secondary response (re-exposure):

• Lag phase: 1-2 days (memory cells are already present)
• IgG dominates (much higher levels than primary IgM)
• Much higher total antibody titer
• Higher antibody affinity (somatic hypermutation and affinity maturation have occurred in germinal centers)
• Longer duration
• This is why vaccination works — memory B and T cells are primed and ready

T Lymphocytes and Cell-Mediated Immunity

T cells develop in the thymus and recognize antigen presented by MHC (major histocompatibility complex) molecules on APCs. T cells cannot recognize free/native antigen — they only recognize processed peptides bound to MHC.

CD4+ Helper T cells (Th cells): Recognize antigen bound to MHC class II (on APCs — dendritic cells, macrophages, B cells). The "orchestrators" of the immune response.

• Th1: Secretes IFN-γ, activates macrophages (cell-mediated immunity, defense against intracellular pathogens). IFN-γ also promotes B cell class switching to opsonizing IgG.
• Th2: Secretes IL-4, IL-5, IL-13; promotes B cell activation and antibody production; defends against helminths; promotes IgE class switching. Implicated in allergic responses.
• Th17: Secretes IL-17; recruits and activates neutrophils; defends against extracellular bacteria and fungi at mucosal surfaces. Strongly implicated in periodontitis pathogenesis — IL-17 drives neutrophil recruitment and RANKL expression, promoting bone loss.
• Treg (regulatory T cells): Suppress immune responses, maintain self-tolerance, limit tissue damage. Secrete IL-10 and TGF-β. The Th17/Treg balance is thought to be important in determining periodontitis susceptibility.

CD8+ Cytotoxic T cells (Tc/CTL): Recognize antigen bound to MHC class I (on all nucleated cells). Kill virus-infected cells and tumor cells via perforin-granzyme and Fas-FasL pathways. The primary effector cells against intracellular pathogens.

MHC Restriction:

• MHC class I: On all nucleated cells. Presents endogenous (intracellular) antigens → recognized by CD8+ T cells
• MHC class II: On professional APCs (dendritic cells, macrophages, B cells). Presents exogenous (phagocytosed) antigens → recognized by CD4+ T cells

Active vs. Passive Immunity

Oral Mucosal Immunity: Unique Features

The oral cavity presents a unique immunological challenge — it must maintain tolerance to commensal microbiota and dietary antigens while mounting effective defenses against pathogens.

Components of oral mucosal immunity:

• Salivary sIgA: The dominant antibody in saliva, produced by plasma cells in the salivary glands (GALT-like — gut-associated lymphoid tissue-like organization). Provides immune exclusion at mucosal surfaces.
• Gingival crevicular fluid (GCF): A serum transudate/inflammatory exudate containing IgG, complement, neutrophils, and antibodies. IgG levels in GCF increase with inflammation to levels comparable to serum. GCF also contains locally produced antibodies from plasma cells in the gingival connective tissue.
• Langerhans cells: Intraepithelial dendritic cells that are the primary APCs in the oral mucosa. They capture antigens that penetrate the epithelial barrier, migrate to regional lymph nodes, and activate na•ve T cells.
• Oral tolerance: The default response to oral antigens (food, commensals) is tolerance (active suppression of immune responses) rather than immunity. Breakdown of oral tolerance is implicated in food allergies and inflammatory bowel disease.
• Neutrophils: Continuously migrate through the junctional epithelium into the gingival sulcus, forming a "neutrophil wall" between the plaque biofilm and the host tissues.

Immunopathology: Hypersensitivity Reactions

The NBDHE may test the four types of hypersensitivity:

Immunological Conditions with Oral Manifestations

Aphthous stomatitis: Not infectious. Thought to involve T cell-mediated immune dysregulation. Triggers include stress, trauma, sodium lauryl sulfate (SLS), nutritional deficiencies (B12, folate, iron), and certain foods.

Lichen planus: T cell-mediated autoimmune condition targeting basal keratinocytes. Oral lesions are common (reticular, erosive forms).

Pemphigus vulgaris: Type II hypersensitivity — autoantibodies (IgG) against desmoglein 3 (and sometimes desmoglein 1) → loss of keratinocyte adhesion (acantholysis) → intraepithelial blistering. Oral lesions are the presenting sign in >50% of cases.

Sjögren's syndrome: Autoimmune destruction of salivary and lacrimal glands → xerostomia and keratoconjunctivitis sicca. Associated with autoantibodies: anti-Ro/SSA and anti-La/SSB.

Clinical Application

Understanding immunology is directly relevant to dental hygiene practice: (1) explaining vaccine mechanisms; (2) recognizing oral manifestations of immunodeficiency (HIV/AIDS — candidiasis, hairy leukoplakia, Kaposi sarcoma, necrotizing periodontal diseases); (3) understanding why patients with neutropenia or neutrophil dysfunction develop severe periodontitis; (4) recognizing Type I hypersensitivity to latex, local anesthetics, and other dental materials; (5) understanding the immunological basis of oral mucosal diseases (lichen planus, pemphigus, cicatricial pemphigoid); and (6) appreciating that inflammation in periodontitis is driven by the host immune response (IL-1, TNF-α, MMPs, RANKL).

Common Traps

• Confusing the immunoglobulin that crosses the placenta (IgG) with the one in breast milk (IgA)
• Thinking IgM can cross the placenta — it cannot (elevated IgM in cord blood indicates intrauterine infection)
• Thinking IgA is the most abundant in serum — it's IgG (75%). IgA is most abundant in secretions.
• Forgetting that salivary IgA is dimeric (sIgA) with secretory component — different from serum monomeric IgA
• Confusing active natural (infection) with passive natural (maternal antibody transfer)