Antigens and Antibodies

Antigens - Immune System Alarms

• Antigen (Ag): Molecule recognized by immune system, triggering response via antibody (Ab) or T-cell receptor (TCR).
• Properties:
  • Immunogenicity: Ability to induce an immune response.
  • Antigenicity: Ability to bind with Abs/TCRs.
• Epitope (Antigenic Determinant): Specific part of Ag (determinant) that binds Ab/TCR.
  • B-cell epitopes: Surface, linear/conformational.
  • T-cell epitopes: Processed linear peptides + MHC.
• Hapten: Small molecule; antigenic, but immunogenic only when bound to a carrier protein.

  ⭐ Haptens are antigenic but not immunogenic unless carrier-bound (e.g., penicillin).

• Key Immunogenicity Factors: Foreignness, molecular size (>10 kDa), complexity, degradability.
• Adjuvant: Substance enhancing an antigen's immunogenicity (e.g., Alum).

Antibodies - The Bodyguard Brigade

• Antibodies (Immunoglobulins, Ig): Glycoproteins from plasma cells; bind specific antigens, neutralizing or marking for destruction.
• Structure: Y-shaped; 2 identical heavy (H) & 2 light (L) chains linked by disulfide bonds.
  • Fab (Fragment, antigen-binding): Variable region; binds antigen. Valency: number of antigen-binding sites (e.g., IgG: 2).
  • Fc (Fragment, crystallizable): Constant region; effector functions (e.g., complement activation, cell receptor binding).
• 
• 📌 Classes (Isotypes) - GAMED (IgG, IgA, IgM, IgD, IgE):

Ag-Ab Interactions - Binding & Beyond

• Binding: Reversible, non-covalent forces (📌 "VISH H": Van der Waals, Ionic, Hydrophobic, Hydrogen bonds).
• Key Properties:
  • Affinity: Strength of one Ag-epitope + Ab-paratope bond.
  • Avidity: Overall binding strength (multiple sites); functionally more important.
  • Specificity: Precise Ab binding to unique Ag epitope.
  • Cross-reactivity: Ab binds similar epitope on different Ag.
• Reaction Types: Based on Ag nature & detection.

• Precipitation Curve: Visualizes zones.
• Other Major Reactions: Neutralization, Complement Fixation, ELISA, Immunofluorescence.

  ⭐ Prozone phenomenon: Antibody excess ↓ visible reaction (e.g., precipitation/agglutination), leading to false negatives. Small, soluble Ag-Ab complexes form.

Antibody Diversity & Uses - Arsenal & Victory

• Diversity Generation:
  • VDJ Recombination: Random joining of V, D, J gene segments in B-cell precursors.

*   Somatic Hypermutation: Point mutations in variable regions after antigen contact, ↑ affinity.
*   Class Switch Recombination: Changes constant region (isotype), alters effector function.

• Monoclonal Antibodies (mAbs) - Therapeutic Uses:
  • Target specific antigens for therapy (e.g., cancer, autoimmune diseases).
  • 📌 Nomenclature: Suffix indicates origin:
    • -omab: Mouse
    • -ximab: Chimeric
    • -zumab: Humanized
    • -umab: Human

⭐ Rituximab is a chimeric monoclonal antibody targeting CD20, widely used in lymphomas and autoimmune conditions like rheumatoid arthritis.

High‑Yield Points - ⚡ Biggest Takeaways

• Haptens: incomplete antigens, immunogenic with carrier; Adjuvants (Alum) enhance immune response.
• IgG: most abundant serum Ig, crosses placenta, key in secondary response, opsonizes.
• IgM: largest Ig (pentamer), first antibody in primary response, potent complement fixer.
• IgA: main Ig in mucosal immunity (secretions), secretory form is a dimer.
• IgE: mediates allergic reactions (Type I hypersensitivity), defense against helminths.
• Isotype switching (IgM → IgG) requires T-cell help (CD40-CD40L).