Comprehensive Study Guide on the Human Immune System and Oral Immunology

Core Principles and Classification of Immunity

  • Fundamental Principle of Healing:

    • No doctor has ever healed anyone of anything in the history of the world; the human immune system heals and that is the only thing that heals (attributed to Bob Wright).
    • Academic contributor: Dr Mokshada Sharma (MBBS, MSc Public Health, PGDHIVM candidate, PhD candidate).

  • Innate Immunity:

    • Constitutes pre-existing, antigen-non-specific host defenses.
    • Provides rapid, non-specific protection against a broad spectrum of invading pathogens.

  • Acquired (Adaptive) Immunity:

    • Responds in an antigen-specific manner to neutralize threats efficiently.
    • Retains immunological memory of the encounter so that any subsequent exposure to the identical threat elicits an accelerated, heightened protective response.

  • Major Branches and Barrier Defense Overview:

    • Host defense is arranged across progressive layers:
      • Physical and physiological barriers (first line).
      • Innate cellular and chemical mechanisms (second line).
      • Adaptive humoral and cell-mediated responses (third line).

Pathogens interacting with physical and physiological barriers, innate immunity, and adaptive immunity

Physical, Anatomical, and Physiological Barriers

  • Anatomical Barriers:

    • Skin:
      • Stratified squamous epithelia located in the epidermis provide a physical cellular barrier.
      • Sebaceous oil glands secrete unsaturated fatty acids, which serve as a poor source of energy for microbial colonizers and maintain an acidic pH\text{pH}.
      • Sweat physically washes away microbes from the surface.
      • Hair acts as a physical barrier to entrap particles and microbes.
    • Mucous Membranes:
      • Epithelial lining coated with mucus to trap microbial invaders.
      • Mucous secretions, tears, and saliva contain antimicrobial proteins, including defensins and lysozymes.

  • Mechanical Removal:

    • Cilia: Coordinated beat patterns sweep foreign particulates and mucus outward along the respiratory tract.
    • Fluid Flow: Salivary flow, lacrimation, and epithelial desquamation continuously dislodge adherent microbes.

  • Physiological Barriers:

    • Gastric Acid: Low stomach pH\text{pH} destroys swallowed pathogens.
    • Body Temperature: Inhibits the growth parameters of temperature-sensitive pathogens.
    • Microbial Antagonism: Normal commensal microflora suppress pathogen colonization through competition for binding sites and nutrients.

Innate Immunity: Cellular, Chemical, and Physiological Defenses

  • Cellular Defenses:

    • Phagocytes:
      • Neutrophils: Primarily target and phagocytose bacterial pathogens.
      • Macrophages: Differentiated from circulating monocytes into two main groups:
        • Wandering monocytes: Actively migrate through blood and tissues to sites of infection.
        • Fixed cells (tissue-resident macrophages): Histiocytes (located in connective tissue), Kupfa cells (located in the kidney), alveolar cells (located in the lungs), and microglia (located in nervous tissue).
    • Natural Killer (NK) Cells: Cytotoxic innate lymphocytes targeting compromised host cells via specialized secretion granules:
      • Perforins: Polymerize to form functional pores/holes in the target antigen membrane.
      • Granzymes: Enter the target antigen through perforin-induced pores to trigger programmed cell death (apoptosis).
    • Granulocytes and Accessory Immune Cells:
      • Basophils: Granulocytic cells involved in inflammatory and allergic responses.
      • Eosinophils: Granulocytes specialized in combating parasites and participating in hypersensitivity reactions.
      • Mast Cells: Tissue-resident effector cells releasing preformed inflammatory mediators.
      • Dendritic Cells: Professional antigen-presenting cells that sample antigens and bridge innate and adaptive immunity.

  • Chemical Defenses:

    • Complement System: A network of circulating serum complement proteins that support innate immune responses by initiating a sequential chemical production cascade, mediating pathogen lysis, opsonization, and recruitment of phagocytes.
    • Cytokines: Intercellular signaling molecules coordinating the immune response:
      • Interleukins (IL): Primarily mediate communication and activation among white blood cells (WBCs).
      • Interferons (IFN): Provide targeted antiviral defenses against viral replication.
      • Tumor Necrosis Factor (TNF): Mediates acute inflammation, cytotoxicity, and immune regulation.
    • Acute Phase Response: Systemic host response triggering acute-phase reactant synthesis.

  • Physiological Defense Mechanisms:

    • Inflammation: Triggered by damage to vascular tissue to coordinate immune cell recruitment:
      • Prostaglandins: Synthesized and released by damaged tissue cells.
      • Histamines: Released primarily from mast cell storage granules.
      • Bradykinins: Generated directly as a result of vascular tissue damage.
      • Vascular Dynamics: Vasodilation is triggered, accompanied by increased vascular permeability, allowing immune cells to extravasate, congregate at the injury site, and eliminate antigens.
    • Fever:
      • Triggered by exogenous and endogenous pyrogens.
      • Regulated centrally by the hypothalamus to elevate core body temperature, thereby suppressing pathogen replication and accelerating host metabolic defenses.

Adaptive Immunity: Classifications, Cell Types, and Immunoglobulins

  • Types of Acquired Immunity:

    • Active Acquired Immunity: Induced when the host produces its own antibodies:
      • Infection: Natural exposure to live pathogens.
      • Vaccine: Controlled exposure to attenuated pathogens, inactivated toxins, or subunit antigens.
    • Passive Acquired Immunity: Conferred through the receipt of pre-formed antibodies without generating host immunological memory:
      • Monoclonal Antibodies: Direct therapeutic administration (e.g., post-exposure rabies prophylaxis, tetanus treatment).
      • Maternal Antibodies: Natural maternal transfer to offspring across the placenta (IgG) or via breast milk (predominantly secretory IgA).

  • Cellular Mediators of Adaptive Immunity:

    • T Helper Cells (CD4+ T Cells): Express T-cell receptors (TCR) alongside CD4 coreceptors to coordinate cell-mediated and humoral responses.
    • Cytotoxic T Cells (CD8+ T Cells): Express TCR alongside CD8 coreceptors to destroy virus-infected and malignant target cells.
    • Naïve B Cells: Mature lymphocytes bearing membrane-bound B-cell receptors (BCR) awaiting initial antigen recognition.
    • Plasma Cells: Terminally differentiated effector B cells specialized in secreting high volumes of soluble immunoglobulins.

  • Classes and Structural Characteristics of Immunoglobulins:

Structures and functions of immunoglobulin classes: IgA, IgD, IgE, IgG, and IgM

*   **Immunoglobulin A (IgA):**
    *   *Structure:* Dimeric form in secretions (linked via a joining [J] chain and secretory component).
    *   *Properties:* Present in mucous, saliva, tears, and breast milk; provides critical barrier defense by protecting mucosal surfaces against pathogens.
*   **Immunoglobulin D (IgD):**
    *   *Structure:* Monomeric.
    *   *Properties:* Functions primarily as a component of the B-cell receptor on naïve B cells; activates basophils and mast cells.
*   **Immunoglobulin E (IgE):**
    *   *Structure:* Monomeric.
    *   *Properties:* Mediates host defense against parasitic worms (helminths); functions as the primary effector antibody responsible for type I allergic and hypersensitivity reactions.
*   **Immunoglobulin G (IgG):**
    *   *Structure:* Monomeric.
    *   *Properties:* Secreted by plasma cells into peripheral blood; represents the most abundant circulating antibody class; capable of crossing the placenta to confer passive immunity to the fetus.
*   **Immunoglobulin M (IgM):**
    *   *Structure:* Membrane-bound monomer on B-cell surfaces; secreted pentamer (five basic antibody units linked by a central J-chain).
    *   *Properties:* Attached to naïve B-cell surfaces as a receptor or secreted into the bloodstream; serves as the primary antibody produced in early stages of immunity.

Oral Cavity Immunology: Innate and Adaptive Mucosal Defenses

  • Non-Specific Host Defence Factors of the Mouth (Table 8.3):

Non-specific host defence factors of the mouth and their main functions

*   *Epithelial Desquamation:* Drives continuous physical removal of adherent microbes from oral surfaces.
*   *Saliva Flow:* Mediates mechanical clearance and physical flushing of microflora.
*   *Mucin / Agglutinins:* Induce bacterial clumping to facilitate physical clearance of microbes.
*   *Lysozyme:* Hydrolyzes bacterial cell walls, inducing cell lysis (bactericidal and fungicidal activity).
*   *Lactoferrin:* Withholds essential iron via iron sequestration, producing bactericidal and fungicidal effects.
*   *Apolactoferrin:* Iron-depleted form of lactoferrin; mediates bactericidal and fungicidal activity via iron sequestration.
*   *Sialoperoxidase System:* Generates antimicrobial hypothiocyanite at neutral pH\text{pH} and hypocyanous acid at low pH\text{pH}.
*   *Histidine-Rich Peptides:* Exert targeted antibacterial and antifungal activities.
*   *Salivary Leukocyte Protease Inhibitor (SLPI):* Blocks host cell surface receptors required for human immunodeficiency virus (HIV) cellular entry.
*   *Intraepithelial Lymphocytes and Langerhans Cells:* Provide an organized cellular barrier against penetrating bacteria and foreign antigens.
*   *Secretory IgA (s-IgA):* Prevents microbial adhesion to mucosal surfaces and inhibits microbial metabolism.
*   *IgG, IgA, IgM:* Inhibit microbial adhesion, function as opsonins to enhance phagocytosis, and act as complement activators.
*   *Complement:* Triggers activation and chemotactic recruitment of neutrophils.
*   *Neutrophils / Macrophages:* Perform active phagocytosis of invading microbial targets.
  • Oral Adaptive Immunity and Lymphoid Architecture:
    • Oral Lymphoid Tissues:
      • Palatine and Lingual Tonsils: Components of the nasopharynx-associated lymphoid tissue.
      • Salivary Gland Lymphoid Tissue: Major local contributor to secretory IgA (S-IgA) synthesis.
      • Gingival Lymphoid Tissue: Localized mucosal immune cells in the gingival crevice.
      • Submucosal Lymphoid Cells: Dispersed cellular infiltrates providing diffuse immune surveillance.
    • Mechanisms of Secretory IgA (s-IgA):
      • Binds directly to surface antigens of microbes within oral fluids.
      • Agglutinates pathogens, preventing them from adhering to mucosal epithelial surfaces.
      • Promotes clearance via salivary flow and swallowing, minimizing tissue invasion.

Anatomical compartmentalization of innate and adaptive immune factors in the oral cavity

  • Anatomical Compartmentalization of Oral Immune Defenses:
    • Oral Mucosa:
      • Toll-like receptors (TLRs)
      • Secretory IgA (SIgA)
      • β\beta-defensins
      • Calprotectin
      • Mucus coat
      • Intraepithelial lymphocytes
      • Lamina propria lymphocytes
      • Microbial antagonism
    • Saliva and Salivary Glands:
      • SIgA
      • Histatins and cystatins
      • Lactoferrin
      • Lysozyme
      • Thrombospondin-1
      • Peroxidase activity
      • Mucins and agglutinins
      • Salivary leukocyte protease inhibitor (SLPI)
      • Chromogranin A
    • Nasopharynx-Associated Lymphoid Tissue (NALT / Tonsils):
      • SIgA
      • CD4+ T cells
      • B cells
      • Transforming growth factor-beta (TGF-β\text{TGF-}\beta)
      • T helper 17 (Th17) response
    • Tooth Plaque Fluid and Adjacent Gingival Crevicular Fluid (GCF):
      • Calprotectin
      • Thrombospondin-1
      • Polymorphonuclear leukocytes (PMNL) and their lysosomal contents
      • Complement components
      • Enamel pellicle
      • Immunoglobulins (IgM, IgG)
      • Peroxidase activity
      • Lysozyme
    • Tongue:
      • Circumvallate papillae
      • Microbial antagonism
      • von Ebner's protein
      • von Ebner's glands
      • Normal commensal microflora