Overview
This lecture covers the five main antibody classes, their structures, functions, and roles in immune responses, as well as antibody structure and the concepts of passive and active immunity.
The Five Antibody Classes (GAMED)
- Use the mnemonic "GAMED" to remember the five classes: IgG, IgA, IgM, IgE, IgD.
- All antibodies are secreted mainly by plasma cells.
IgG Antibody
- IgG is the most abundant antibody (65-70% in blood plasma).
- Exists as a monomer and is found in blood and extracellular fluids.
- Produced in both primary and especially secondary immune responses.
- Functions: activates complement, opsonization, neutralization, precipitation, and crosses the placenta for passive immunity.
IgA Antibody
- IgA is mainly a dimer (two linked monomers) found in body fluids (saliva, sweat, GI mucosa, milk, urogenital tract).
- Provides immunity at mucosal surfaces and via breastfeeding to infants.
IgM Antibody
- IgM exists as a pentamer (five units) or monomer.
- First antibody produced in primary immune response; abundant in early infection.
- Activates complement, causes agglutination (especially in mismatched blood transfusions), and has 10 antigen binding sites.
- Monomeric IgM can function as a B cell receptor.
IgE Antibody
- IgE is a monomer found in respiratory tract mucosa, urogenital tissues, and some blood.
- Binds to mast cells/basophils, triggers release of histamine and other mediators (type 1 hypersensitivity/anaphylaxis).
- Important in responses to allergens and parasites (eosinophil recruitment).
IgD Antibody
- IgD is a monomer expressed on B cell surfaces as a receptor (not usually secreted).
- Functions as a B cell receptor alongside monomeric IgM.
Antibody Production and Responses
- Primary immune response: IgM produced first, then IgG.
- Secondary immune response: IgG dominates due to somatic hypermutation (switch from IgM to IgG).
Passive and Active Immunity
- Passive immunity: receiving antibodies (naturally—placenta/milk; artificially—antivenom).
- Active immunity: producing antibodies after exposure (naturally—infection; artificially—vaccines).
Antibody Structure
- Antibodies consist of two heavy chains and two light chains (constant and variable regions).
- Variable regions determine antigen-binding specificity.
- Each antibody has two antigen binding sites; IgM pentamer has ten total.
- Disulfide bonds hold chains together; the Fc region binds complement proteins.
Key Terms & Definitions
- Antibody/Immunoglobulin — Protein made by plasma cells that recognizes and binds antigens.
- Plasma Cell — A differentiated B cell that secretes antibodies.
- Monomer/Dimer/Pentamer — Antibody forms with one, two, or five basic units, respectively.
- Opsonization — Tagging pathogens for phagocytosis.
- Complement System — Proteins that aid antibody-mediated destruction of pathogens.
- Agglutination — Clumping of cells (e.g., RBCs) by antibodies.
- Somatic Hypermutation — Genetic changes in B cells increasing antibody affinity during secondary response.
- Passive Immunity — Immunity from transferred antibodies, not self-produced.
- Active Immunity — Immunity from self-production of antibodies after exposure.
Action Items / Next Steps
- Review antibody class functions and structures.
- Study antibody response graphs (primary vs. secondary).
- Read textbook sections on humoral immunity and hypersensitivity types.