Humoral Immunity Quiz

Humoral Immunity (HI)

• Definition of Antibody: Glycoproteins found in serum and tissue fluids, binding specifically to the antigen that triggered their production.
• Humoral Immunity: Mediated by antibodies produced by B lymphocytes and secreted by plasma cells.
• B Cell Activation:
  • T cell dependent activation: Antigen is protein-based, B cells act as antigen-presenting cells (APCs), requiring T lymphocytes for activation.
  • T cell independent activation: Antigen is non-protein based, these molecules are not processed by APCs and do not activate helper T cells.
• Structure of Immunoglobulins:
  • Basic unit (monomer): Four polypeptide chains – two light chains and two heavy chains.
  • Light chains: Two antigen types: Kappa (K) and Lambda (A). Only one type found in an individual molecule.
  • Heavy chains: Five types: Gamma (γ), Alpha (α), Mu (µ), Delta (δ), and Epsilon (ε).
  • Disulfide Bonds: Connect between heavy and light chains, the two heavy chains, and within polypeptide chains.
• Variable (V) and Constant (C) Regions:
  • Light chain: Variable (VL) and Constant (CL)
  • Heavy chain: Variable(VH) and 3 Constant (CH)
• Hinge Region: Where the arms of the antibody molecule form a Y shape.
  • Fab: Fragment containing antigen binding sites.
  • Fc: Fragment containing the remainder of the two H chains (mediates effector functions).

Functions of Immunoglobulins

• Antigen Binding: Specific binding to antigens.
• Activation of Complement: Leads to cell lysis.
• Opsonization: Coating of organisms with antibodies, enhancing phagocytosis.
• Antibody-dependent Cellular Cytotoxicity (ADCC): Destruction of antibody-coated target cells by NK cells, macrophages, neutrophils, eosinophils.

Classes of Immunoglobulins

• IgG:

  • Gamma heavy chains
  • Structure: Monomeric unit
  • Properties & Functions:
    • Major serum immunoglobulin (75% of serum Ig).
    • Major Ig in secondary immune response.
    • Only Ig that crosses the placenta.
    • Activates complement.
    • Opsonization.
    • Rh antibodies are IgG.

• IgM:

  • Mu heavy chains
  • Structure: Pentamer with a J chain (monomer on B lymphocytes as an antigen receptor).
  • Properties & Functions:
    • Third most common serum Ig (7% of serum Ig).
    • Major Ig in primary immune response.
    • Cannot cross the placenta.
    • Activates complement.
    • Good agglutinating Ig.
    • Antibodies of ABO blood groups.

• IgA:

  • Alpha heavy chains
  • Structure: Serum IgA - monomer; Secretory IgA in secretions - dimer with a J chain and secretory piece.
  • Properties & Functions:
    • Second most common serum Ig (15% of serum Ig).
    • Secretory IgA present in secretions (tears, saliva, colostrum, mucus), essential for mucosal immunity.
    • Inhibits binding of pathogens to mucosal cells.

• IgD:

  • Delta heavy chains
  • Structure: Monomer
  • Properties & Functions:
    • Found in low levels in serum.
    • Found on B cell surfaces as an antigen receptor.

• IgE:

  • Epsilon heavy chains
  • Structure: Monomer
  • Properties & Functions:
    • Least common serum Ig, binds tightly to Fc receptors on basophils and mast cells.
    • Involved in Type I hypersensitivity reactions.
    • Plays a role in parasitic infections.
    • Eosinophils have Fc receptors for IgE, leading to killing of parasites through ADCC.

Primary Immune Response

• First exposure to a foreign antigen.
• Lag phase of 1-2 weeks for detectable antibody levels (mainly IgM).
• Antibody levels decline rapidly.

Secondary Immune Response

• Second exposure to the same antigen after primary immunity.
• Rapid and steep rise in antibody levels (mainly IgG) that persists for weeks or months.
• Due to the presence of immunological memory cells.
• Faster and stronger response than the primary response.

Monoclonal Antibodies

• Definition: Highly specific antibodies produced against a single epitope.
• Formation: Obtained by fusing a B cell producing antibodies against a single epitope with a myeloma cell (malignant plasma cell). The fused cell is called a hybridoma cell.