Detailed Notes on Antigen Recognition in the Adaptive Immune System

Antigen Recognition in the Adaptive Immune System

Immunoglobulin Superfamily

  • Ig domains are found in many immune system proteins and proteins outside the immune system.
  • These proteins often respond to environmental and cellular stimuli.
  • Proteins with Ig domains belong to the immunoglobulin superfamily, possibly evolved from a common ancestral gene.

Antibody Structure and Function

  • Antigen-binding site: Located in the variable (V) regions of the heavy (H) and light (L) chains.
  • Core antibody structure contains two identical antigen-binding sites.
  • Variable regions: The heavy chain (VH) and light chain (VL) contain hypervariable regions or CDRs.
  • CDRs (Complementarity Determining Regions):
    • Three CDRs exist in each VH and VL.
    • CDR3 exhibits the most variability and is at the V and C region junction.
    • CDR3 contributes the most to antigen binding.
  • Fab (fragment, antigen-binding):
    • Contains a whole light chain (V and C domains) attached to the V and first C domains of a heavy chain.
    • Required for antigen recognition.
  • Fc (fragment, crystalline) region:
    • Composed of remaining heavy-chain C domains.
    • It tends to crystallize in solution.
    • Responsible for the antibody's biologic activity and effector functions.
  • Each Ig molecule has two identical Fab regions for antigen binding and one Fc region for biologic activity.
  • Some antibodies exist as multimers (two to five antibody molecules attached).
  • Hinge region:
    • A flexible portion between the Fab and Fc regions.
    • Allows independent movement of the two Fab regions.
    • Enables simultaneous binding to antigen epitopes separated by varying distances.
  • The C-terminal end of the heavy chain:
    • May be anchored in the plasma membrane (B cell receptors).
    • May terminate in a tail piece for secreted antibodies.
  • Light chains are not attached to cell membranes.
  • Antibodies (Immunoglobulins - Igs): Immunity-conferring proteins with the electrophoretic mobility of globulins.
    • They neutralize and eliminate microbes and toxins in mucosal secretions (effector molecules of humoral immunity).

Antibody Function in Immune Responses

  • Secreted antibodies recognize microbial antigens and toxins through variable domains.
  • Constant regions of secreted antibodies bind to molecules like phagocyte receptors and complement system proteins.
  • Functions at different stages:
    • Membrane-bound antibodies on B cells recognize antigens to initiate immune responses.
    • Secreted antibodies neutralize and eliminate microbes and their toxins in the effector phase.
  • In cell-mediated immunity:
    • T lymphocytes and other leukocytes perform the effector function of microbe elimination.
    • T cell receptors are only involved in antigen recognition and T cell activation; they are not secreted and do not mediate effector functions.

Antibody Molecule Composition

  • Composed of four polypeptide chains:
    • Two identical heavy (H) chains.
    • Two identical light (L) chains.
    • Each chain has a variable and a constant region.
  • The four chains assemble into a Y-shaped molecule.
  • Each light chain attaches to one heavy chain, and heavy chains attach to each other via disulfide bonds.
  • A light chain comprises one V and one C domain; a heavy chain has one V and three or four C domains.
  • Each domain folds into an immunoglobulin domain.
  • Immunoglobulin domain:
    • Two layers of β-pleated sheet held together by a disulfide bridge.
    • Adjacent strands connected by short, protruding loops.
    • In Ig molecules, these loops form the three CDRs.

Affinity and Avidity

  • Affinity: The strength of binding between one antigen-binding surface of an antibody and one epitope of an antigen.
    • Expressed as the dissociation constant (Kd)(K_d).
    • KdK_d: Molar concentration of antigen required to occupy half the available antibody molecules in a solution.
    • Lower KdK_d = higher affinity.
    • Primary immune response: KdK_d in the range of 10−610^{-6} to 10−910^{-9} M.
    • Secondary immune response (repeated stimulation): affinity increases to a KdK_d of 10−810^{-8} to 10−1110^{-11} M, known as affinity maturation.
  • IgG, IgD, and IgE have two antigen-binding sites.
  • Secreted IgA is a dimer with four antigen-binding sites.
  • Secreted IgM is a pentamer with 10 antigen-binding sites.
  • Each antibody molecule can bind 2 to 10 epitopes of an antigen or epitopes on neighboring antigens.
  • Avidity: The total strength of binding, greater than the affinity of a single antigen-antibody bond.
  • Cross-reaction: Antibodies produced against one antigen may bind other, structurally similar antigens.

B Cell Receptor Complex

  • In B lymphocytes, membrane-bound Ig molecules are noncovalently associated with Igα and Igβ.
  • The three proteins form the BCR complex.
  • When the BCR recognizes antigen, Igα and Igβ transmit signals to activate the B cell.

Monoclonal Antibodies

  • Exploitation of the fact that one clone of B cells makes an antibody of only one specificity. They are a major advancement for clinical medicine and research.
  • Produced by fusing B cells from an immunized animal with myeloma cells (plasma cell tumors).
  • Myeloma cells can be propagated indefinitely in tissue culture.
  • The myeloma cell line lacks a specific enzyme, preventing growth in a toxic drug, but fused cells can grow.

Light and Heavy Chain Types

  • Light chains:
    • Two types: κ and λ, differing in their C regions.
    • Each B cell expresses either κ or λ, but not both.
  • Heavy chains:
    • Five types: µ, δ, γ, ε, and α, differing in their C regions.
    • Each light chain can complex with any heavy chain.
  • Isotypes (classes): Antibodies with different heavy chains (IgM, IgD, IgG, IgE, and IgA).
    • Each isotype has distinct physical, biologic properties, and effector functions.
  • Antigen receptors of naive B lymphocytes are membrane-bound IgM and IgD.
  • After stimulation by antigen and helper T lymphocytes:
    • The antigen-specific B lymphocyte clone expands and differentiates into antibody-secreting progeny.
    • Progeny may secrete IgM or switch to other heavy-chain classes.
  • Heavy-chain class (isotype) switching:
    • The mechanism and importance are discussed in Chapter 7.
    • Occurs during humoral immune responses.
    • Each clone of B cells maintains its specificity because the V regions do not change.
    • The light-chain class (κ or λ) also remains fixed throughout the life of each B cell clone.

Antigen Binding

  • Antibodies bind a wide variety of antigens, including macromolecules and small chemicals.
  • The antigen-binding CDR loops of antibody molecules can form clefts for small molecules or flat surfaces for larger molecules.
  • Antibodies bind to antigens through reversible, noncovalent interactions:
    • Hydrogen bonds.
    • Hydrophobic interactions.
    • Charge-based interactions.
  • Epitopes (determinants): Parts of antigens recognized by antibodies.
    • Linear epitopes are based on the sequence of a stretch of amino acids.
    • Conformational epitopes are based on the shape.
    • Some epitopes are hidden within antigen molecules and are exposed by physicochemical changes.

Antibody Isotypes

  • Classified by heavy (H) chains; each isotype may contain either κ or λ light chain.
  • IgA consists of two subclasses: IgA1 and IgA2.
  • IgG consists of four subclasses: IgG1, IgG2, IgG3, and IgG4.
Isotype-Specific Features:
  • IgA:
    • Subtypes: IgA1,2
    • H chain: α (1 or 2)
    • Plasma concentration: 3.5 mg/mL
    • Plasma half-life: 6 days
    • Secreted form: Monomer, dimer
    • Functions: Mucosal immunity
  • IgD:
    • H chain: δ
    • Plasma concentration: Trace
    • Plasma half-life: 2 days
    • Secreted form: Monomer
    • Functions: Naive B cell antigen receptor
  • IgE:
    • H chain: ε
    • Plasma concentration: 0.05 mg/mL
    • Plasma half-life: 23 days
    • Secreted form: Monomer
    • Functions: Mast cell activation (immediate hypersensitivity), Defense against helminthic parasites
  • IgG:
    • Subtypes: IgG1-4
    • H chain: γ (1,2,3 or 4)
    • Plasma concentration: 13.5 mg/mL
    • Plasma half-life: 23 days
    • Secreted form: Monomer
    • Functions: Opsonization, complement activation, antibody-dependent cell-mediated cytotoxicity, neonatal immunity, feedback inhibition of B cells
  • IgM:
    • H chain: µ
    • Plasma concentration: 1.5 mg/mL
    • Plasma half-life: 5 days
    • Secreted form: Pentamer
    • Functions: Naive B cell antigen receptor, complement activation