Overview
This lecture reviews non-specific immunity and antigen nature, then introduces antibody structure, function, and immunoglobulin classes essential for understanding immune responses and clinical applications.
Review of Non-Specific Immunity & Phagocytosis
- Phagocytosis steps: contact, ingestion, phagosome formation, lysosome fusion, digestion, and release.
- Opsonins (complement proteins, CRP, IgG) enhance phagocyte contact with targets.
- Key phagocyte disorders: Chronic granulomatous disease (NADPH defect), glucose-6-phosphate deficiency, myeloperoxidase deficiency, and Chediak-Higashi syndrome.
- NK cells use antibody-dependent cell-mediated cytolysis (ADCC) to kill without MHC restriction.
- Acute phase reactants (e.g. CRP, haptoglobin, fibrinogen, alpha-1-antitrypsin, ceruloplasmin, serum amyloid A) are nonspecific markers of inflammation.
- Cytokines regulate immune activity; key examples include IL-1 (induces IL-6/IL-2), TNF-α (apoptosis), and IL-6 (CRP, plasma cell stimulation).
Antigens and Epitopes
- Autoantigens: self; alloantigens: same species; heteroantigens: different species.
- Antigen = large molecule; epitope = specific binding site on antigen.
- Traits of good antigens: large size, complexity, foreignness, and ability to be processed.
- Haptens are small molecules that become immunogenic only when coupled to carriers.
Antibody (Immunoglobulin) Structure & Terminology
- Antibodies are glycoproteins made by plasma cells; cannot destroy targets, but neutralize pathogens.
- Key regions: variable (FAB, binds antigen/epitope), constant (FC, binds host cell receptors).
- Structure: two heavy chains (define class: gamma, mu, alpha, delta, epsilon), two light chains (kappa or lambda, never mixed in one antibody).
- Constant regions = FC; variable regions = FAB, determines specificity.
- Hinge region allows conformational flexibility for antigen binding.
Antibody Classes & Functions
- IgG: most abundant, crosses placenta, secondary response, fixes complement, valence 2.
- IgM: pentamer in plasma, early/primary response, most efficient at complement activation, valence 10.
- IgA: dimer in secretions (with J-chain and secretory component), mucosal defense, can be monomer in serum.
- IgD: present on B cell surface; function largely unknown.
- IgE: lowest serum concentration, allergies/hypersensitivity, binds mast cells triggering histamine release.
Laboratory and Clinical Relevance
- Monoclonal antibodies: produced by single plasma cell clone (all same idiotype/kappa or lambda), important in lab diagnostics.
- Polyclonal response produces antibody diversity; monoclonal gammopathy (e.g., multiple myeloma) is pathologic.
- Enzymes (papain, pepsin) digest antibodies for diagnostic purposes.
- Blood transfusion reactions arise from antibodies to RBC antigens (especially ABO, Duffy, etc.).
- Measuring immunoglobulins helps diagnose immunodeficiencies and gammopathies.
Key Terms & Definitions
- Phagocytosis — process where cells engulf and digest particles.
- Opsonin — molecule that enhances phagocytosis.
- Epitope — specific site on antigen bound by antibody.
- Fab region — antigen-binding fragment of antibody.
- Fc region — constant region binding host cell receptors.
- Isotype — class of antibody defined by heavy chain.
- Allotype — genetic variants within the same species.
- Idiotype — variable region determining antigen specificity.
- Monoclonal antibody — antibody from a single cell clone.
- J-chain — protein joining immunoglobulin monomers (IgM, IgA).
- Secretory component — protein protecting IgA in secretions.
Action Items / Next Steps
- Review antibody structure and class functions for upcoming test.
- Complete antibody lab (questions due in one week).
- Preview lab questions on Canvas; answer independently.
- Bring questions to next class or email instructor if clarification needed.