Cells & Immunity Notes

Overview of Antibodies

• Antibodies (Abs): Immune proteins produced by B lymphocytes.

• Each individual can generate approximately 10^8 different antibody specificities.

Structure of Antibodies

• Immunoglobulins (Ig) consist of:

  • 3 hypervariable regions located within each Variable Heavy (VH) and Variable Light (VL) domain.

  • Constant regions, with various forms depending on class.

• Light Chains: Can be of kappa (k) or lambda (l) type.

• Heavy Chains: Determine the class of the antibody (IgA, IgD, IgE, IgG, IgM).

  • Biological Roles of Antibody Classes:

    • Each antibody class serves different roles and locations in the immune response.

Diversity of Antibodies

• Gene Segments: Antibody diversity arises from multiple gene segments:

  • Light chains encoded by 3 gene segments.

  • Heavy chains encoded by 4 gene segments.

• All antibodies produced are thus derived from a limited number of genes (few hundred Ig genes).

Classes of Antibodies

• 5 Classes of Antibodies (Isotypes):

  • IgM: Major in primary immune response, confined to the vascular system.

  • IgG: Major in secondary immune response, can cross capillary walls.

  • IgA: Secreted in mucosal areas (saliva, gut, breast milk).

  • IgD: Primarily acts as a receptor on B cells.

  • IgE: Involved in allergic reactions; found bound to mast cells.

Immune Response Dynamics

• Primary vs. Secondary Response:

  • Primary Response: Initial exposure to antigen; slower and produces lower antibody concentration.

  • Secondary Response: Faster and stronger due to memory cells; greater antibody production.

Mechanisms of Antibody Action

• Agglutination: Clumping of antigens (e.g., bacteria) facilitating phagocytosis.

• Opsonization: Tagging antigens with antibodies increases phagocyte efficiency.

• Neutralization: Antibodies bind toxins and viruses, preventing harm.

• Activation of Complement: Triggers a series of immune responses to enhance pathogen destruction.

• ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity): Engages natural killer cells to eliminate infected cells.

Physiological Roles of Antibodies

• Neutralization of Toxins: Blocking the active sites of toxins prevents their harmful effects.

• Preventing Microbe Attachment: Antibodies can block microbial adherence to host cells.

Passive Immunity

• Transfer of Antibodies: Maternal antibodies (IgG) transferred across the placenta or via breast milk (IgA) to protect the infant.

Conclusion on Biological Roles of Antibodies

• Specific Binding to Antigens: The primary role of all antibody classes.

• Various Secondary Roles include agglutination, opsonization, neutralization, complement activation, and providing passive immunity to offspring.

Notable Points

• Serological Profile: Differences in antibody concentration and types during immune responses (e.g., IgM vs. IgG).

• Ig Subclasses: IgG has four subclasses (IgG1, IgG2, IgG3, IgG4) which vary in their immune functions.