Comprehensive Notes: Antigen & Antibody

Antigen and Epitope

  • Antigen (Ag) = substance specifically bound by an antigen-recognizing molecule.
  • Antibodies can recognize a wide range of molecules: metabolites, sugars, lipids, carbohydrates, phospholipids, nucleic acids, and proteins.
  • Epitope (antigenic determinant) = the specific region of the antigen recognized by an antibody.
  • In an antibody–antigen reaction, the epitope complements the paratope perfectly.

Hapten and Hapten-Carrier Conjugates

  • Hapten = a small molecule that can induce an immune response when bound to a carrier molecule (foreign protein).
  • Examples of haptens: dinitrophenyl, aminobenzene, sulphonate, arsonate.
  • Hapten-carrier conjugate acts as an immunogen, whereas the free hapten does not.
  • Carrier protein provides the necessary immunogenic context for the hapten to elicit antibodies against the hapten’s chemical shape.

Immunogenicity vs Antigenicity

  • Immunogenicity = ability of a substance (antigen or epitope) to provoke an immune response in a host.
  • Small molecules by themselves are not antigenic; they become antigenic when bound to a carrier molecule.
  • Antigenicity = the strength of an immune response that an antigen can elicit.
  • Immunogens are always antigens, but not all antigens are immunogenic.
  • Characteristics that help an substance become an immunogen:
    • Foreign substance (organic or inorganic)
    • Chemically complex (heterogeneity in building blocks; proteins vs polysaccharides)
    • High molecular weight (typically >10\,\text{kDa}) – high immunogenic potential
    • Adjuvant presence (substances that enhance immune response to an antigen)

Chemical Nature of Immunogens

  • Major classes:
    • Proteins/polypeptides
    • Polysaccharides
    • Nucleic acids
    • Lipids

Antigen-Recognizing Molecules

  • Three main classes that recognize foreign antigens:
    • Antibodies (immunoglobulins)
    • T cell receptor (TcR)
    • Major histocompatibility complex (MHC) molecules
  • Antibodies can act as membrane-bound receptors during the recognition phase of an immune response and can also be secreted by plasma cells to bind antigen and trigger effector responses.

Antibodies (Immunoglobulins)

  • Antibodies are globular glycoproteins.
  • They consist of heavy chains and light chains, held together by disulfide bonds.
  • Each antibody has 2 identical antigen-binding sites (variable regions) that determine the binding specificity.
  • The order of amino acids in the variable region determines the shape of the antigen-binding site.
  • Antibodies can bind epitopes on a wide range of intact molecules: proteins, carbohydrates, lipids.
  • Antibodies can be membrane-bound receptors on B cells or secreted by plasma cells.
  • Structure overview: a Y-shaped molecule consisting of 4 polypeptide chains (2 heavy chains, 2 light chains) joined by disulfide bonds; includes a variable region at the tips and constant regions in the stem.
  • The heavy and light chains form the antigen-binding sites, with variability concentrated in the variable regions.

Immunoglobulin Isotypes/Classes

  • Classes (isotypes) and subclasses (subtypes) differ in the amino acid sequences of the heavy-chain constant (C) regions.
  • Heavy chains are denoted by Greek letters: α (IgA), μ (IgM), δ (IgD), ε (IgE), γ (IgG).
  • Five classes isotypes: IgG,IgA,IgM,IgE,IgD\text{IgG}, \text{IgA}, \text{IgM}, \text{IgE}, \text{IgD}

IgA and IgM Architecture

  • IgM is typically pentameric with a J chain; allows high avidity due to multiple binding sites.
  • IgA is often dimeric in secretions (also linked by a J chain).
  • IgM binding sites: 1010 per pentamer (i.e., 1010 antigen-binding sites total) due to 5 antibody units per pentamer.
  • IgG generally monomeric; largely responsible for systemic immunity.

Antibody Binding Site Characteristics

  • IgG: 22 antigen-binding sites; functions include crossing the placenta, opsonization, agglutination, and complement activation.
  • IgM: 1010 binding sites; highly effective at agglutination and complement activation.
  • IgA: 22 or 44 binding sites; predominant in secretions (saliva, tears, mucosal surfaces) to guard entrances of the body.
  • IgE: 22 binding sites; acts in tissues; defends against parasites and mediates allergic responses via mast cell activation.
  • IgD: 22 binding sites; part of the B cell receptor complex; initiates early B cell responses and activates basophils/mast cells.

Epitopes and Paratopes

  • Epitope (antigenic determinant) = the part of the antigen recognized by an antibody.
  • Epitopes can be:
    • Covalent/linear determinants (continuous sequence of amino acids)
    • Conformational determinants (discontinuous; formed by 3D folding bringing residues together)
  • Paratope = the antibody binding region that recognizes the epitope.
  • A paratope may bind a linear determinant of >6 amino acids.
  • Some linear determinants are hidden in the native folded protein and only become accessible when the protein denatures (externalized).

Antibody–Antigen Interactions

  • Binding is governed by non-covalent interactions:
    • Hydrogen bonds
    • Electrostatic attractions
    • Van der Waals forces
    • Hydrophobic interactions
  • These interactions are individually weak but collectively form a stable, reversible antibody–antigen complex.

General Functions of Antibodies

  • Block toxins and neutralize antigens.
  • Immobilize bacteria by binding to their flagella.
  • Agglutinate bacteria to limit spread.
  • Activate the complement system.
  • Act as opsonins to enhance phagocytosis.

Antigen Presentation and MHC Overview

  • Major Histocompatibility Complex (MHC) = group of genes involved in the recognition of self vs non-self in animal species.
  • In humans, the MHC is termed Human Leukocyte Antigen (HLA); in mice, H-2.
  • MHC genes are highly polymorphic and polygenic, contributing to transplant rejection risk and individual immune variability.

MHC Classes I and II

  • MHC Class I: expressed on all nucleated cells; presents endogenous peptide antigens to cytotoxic T cells (CD8+ T cells).
  • MHC Class II: expressed on antigen-presenting cells (APCs) like macrophages, B cells, dendritic cells; presents exogenous peptide antigens to helper T cells (CD4+ T cells).
  • Location and composition:
    • Class I: composed of α and β2-microglobulin chains; peptide-binding cleft formed by α1 and α2 domains.
    • Class II: comprised of α and β chains with peptide-binding cleft formed by α1 and β1 domains.
  • Human MHC isotypes: Class I = HLA-A, HLA-B, HLA-C; Class II = HLA-DR, HLA-DQ, HLA-DP.

APCs (Antigen-Presenting Cells)

  • Definition: cells capable of capturing, processing, and presenting antigenic peptides to helper T cells, delivering co-stimulatory signals to achieve proper T cell activation.
  • Primary APCs for naive T cell activation: Dendritic cells, macrophages, B cells.
  • APCs present processed antigen via MHC to T cells and provide co-stimulation (e.g., B7 to CD28).

Antigen-Presenting Cell Pathway and Interactions

  • Antigen-presenting cell processes antigen and presents peptides on MHC molecules to T cells:
    • Dendritic cells: key in initiating T cell responses in lymphoid tissues.
    • Macrophages: present antigen and activate effector T cells; also participate in cell-mediated immunity.
    • B cells: present antigen and receive help from T cells to differentiate and produce antibodies.
  • Naive T cell activation leads to clonal expansion and differentiation into effector T cells; effector T cells mediate various immune responses.

T Cell Receptor (TcR)

  • TcR is a surface molecule analogous to a component of antibody; diversity arises from rearrangement of the TcR gene locus.
  • TcR primarily recognizes peptide antigens derived from pathogens.
  • TcR is antigen-specific and comes in two major functional types:
    • CD4+ T cells (helper T cells)
    • CD8+ T cells (cytotoxic T cells)

CD4+ T Cells (Helper T Cells) vs CD8+ T Cells (Cytotoxic T Cells)

  • CD4+ T cells express CD4 and recognize exogenous antigens presented by MHC Class II on APCs.
  • Exogenous antigens = proteins produced outside cells (bacteria, viruses, protozoa, fungi, parasites).
  • CD4+ T cells secrete cytokines to modulate:
    • B cell activation into plasma cells and antibody production
    • Activation of macrophages and dendritic cells
    • Cellular chemotaxis and inflammation
  • CD8+ T cells express CD8 and recognize endogenous antigens presented by MHC Class I on infected host cells.
  • Endogenous antigens include proteins from inside the cell (e.g., viral proteins or tumor antigens).
  • Mechanism of killing: release perforin and granzymes to create pores in the target cell membrane, leading to cell lysis or apoptosis.

T Cell Subsets and Cytokine Profiles (Extra Information)

  • Th1: IFN-γ, T-bet; promoted by IL-12 and IFN-γ; supports cell-mediated immunity.
  • Th2: Gata3; IL-4, IL-5, IL-13; supports humoral immunity and allergic responses.
  • Th17: RORC2; IL-6, TGF-β; produces IL-17A, IL-17F, IL-22; involved in inflammatory responses.
  • Th22: AhR; IL-22; various regulatory roles on mucosal surfaces.
  • Tfh (T follicular helper): IL-21; supports B cell help in germinal centers.
  • Treg (Regulatory T cells): Foxp3; IL-10 and TGF-β; suppressor of immune responses to maintain tolerance.

Pathways of Antigen Presentation

  • Presentation Pathways involve processing of antigens and loading onto MHC:
    • Class I pathway: endogenous peptide antigens presented to CD8+ cytotoxic T cells.
    • Class II pathway: exogenous peptide antigens presented to CD4+ helper T cells.
  • Antigen processing steps (illustrative): antigen uptake by APC, processing to peptide fragments, loading onto MHC molecules, transport to the cell surface, and recognition by T cell receptors.

Pathway Illustration (Simplified narrative)

  • Infected cell + antigen fragment associates with Class I MHC; TCR on CD8+ T cell recognizes complex leading to cytotoxic response.
  • Antigen-presenting cell engulfs a microbe, digests it in lysosomes, loads antigen fragments onto Class II MHC, and presents to CD4+ T cells; co-stimulation (e.g., CD28–B7) provides full activation.

Exam-Style Prompts (Practice prompts inspired by the transcript)

  • List THREE antigen recognizing molecules.
  • List THREE antigen presenting cells.
  • MHC Class I is expressed on __.
  • MHC Class II is expressed on
  • List all classes of immunoglobulin.
  • State ONE function of antibody.
  • Helper T cell is also known as _.
  • Cytotoxic T cell is also known as _.
  • Fill in the blanks: MHC Class I Characteristics, MHC Class II ?, (a) Types of antigen, Exogenous peptide antigen, Cytotoxic T cell, Responsive T cell, ?, (b) ?, (c) End result, ? (d).

Connections to Foundations and Real-World Relevance

  • Immunogenicity and antigenicity concepts underpin vaccine design and predicting immune responses to new antigens.
  • Hapten-carrier conjugates explain why small molecules can become immunogenic when attached to proteins, informing conjugate vaccines.
  • MHC diversity (polymorphism) explains why some individuals reject transplants while others tolerate grafts; informs donor–recipient matching in transplantation.
  • The distinction between endogenous vs exogenous antigens is central to understanding viral infection vs extracellular bacteria handling and vaccine strategies.
  • Antibody functions (neutralization, opsonization, complement activation) are fundamental to therapeutic antibody development.

Key Formulas and Numbers (summary)

  • Antigen-binding sites (IgG): 22
  • IgM binding sites: 1010 (pentamer)
  • IgA binding sites: 22 or 44
  • Immunogenicity weight threshold: typically greater than 10 kDa10\,\text{kDa} for high immunogenic potential
  • Pentameric IgM structure includes a J chain; dimeric IgA includes a J chain
  • Five immunoglobulin classes: IgG,IgA,IgM,IgE,IgD\text{IgG}, \text{IgA}, \text{IgM}, \text{IgE}, \text{IgD}
  • MHC gene location: chromosome 6 in humans
  • MHC Class I presents to CD8+ T cells; MHC Class II presents to CD4+ T cells

Notes and Cross-References

  • Epitope accessibility can depend on antigen conformation; denaturation may reveal hidden linear determinants.
  • Adjuvants are critical when formulating vaccines to boost immunogenicity of protein or hapten-carrier antigens.
  • APCs deliver necessary co-stimulatory signals to naive T cells; without co-stimulation, T cells may become anergic.
  • The interplay between antibodies and T cell–mediated responses shapes both humoral and cellular immunity in infection and vaccination.