Humoral Response and Antibodies

Classes of Immunoglobulins

Overview of Immunoglobulin Classes

  1. IgM

    • Structure: Pentamer (5 antibodies linked), thus 10 binding sites.

    • Function: First antibody produced in response to an antigen; too large to pass barriers (e.g., placenta).

  2. IgG

    • Structure: Monomer, smaller and thus can cross the placenta to provide fetus immunity.

    • Function: Second antibody class produced, crucial for secondary immune response.

  3. IgA

    • Structure: Dimer (2 linked Y shapes).

    • Function: Most abundant class; critical for mucosal immunity, secreted in breast milk and mucosal membranes.

  4. IgD

    • Function: Less than 1% in serum; role in development of immune response.

  5. IgE

    • Function: Involved in allergic responses or defense against parasites, often undetectable in blood.

Mechanisms of Action upon Antigen Binding

Six Main Responses of Antibodies

  1. Neutralization

    • Antibodies block viral binding sites or toxins, preventing them from attaching to host cells.

  2. Opsonization

    • Antibodies coat pathogens, enhancing phagocyte recognition which makes it easier for phagocytosis.

  3. Activation of the Complement System

    • Antibodies recruit additional immune proteins leading to the destruction of pathogens.

  4. Immobilization and Prevention of Movement

    • Antibodies bind to structures of bacteria (like flagella), rendering them immobile.

  5. Cross-Linking

    • One antibody binds to two bacteria, forming clumps that are easier for phagocytes to engulf collectively.

  6. Antibody-Dependent Cellular Cytotoxicity (ADCC)

    • Infected host cells are coated with antibodies attracting natural killer cells, which induce apoptosis in the infected cell to limit the spread of infection.