Overview
This lecture covers the five major antibody classes (immunoglobulins), their structure, functions, and roles in immune responses, as well as basic concepts of immunity and antibody structure.
Antibody Types Overview (GAMED Mnemonic)
- Five major antibody classes: IgG, IgA, IgM, IgE, and IgD (remembered by "GAMED").
- All antibodies are produced by plasma cells.
IgG Antibodies
- Most abundant antibody, 65-70% found in blood plasma/extracellular fluids.
- Secreted in monomeric form and produced during both primary and (mainly) secondary immune responses.
- Functions: activates complement system, enhances opsonization/phagocytosis, neutralizes pathogens, and can cross placenta for passive immunity to fetus.
IgA Antibodies
- Secreted as a dimer, mainly found in saliva, sweat, mucosal linings (GI, urogenital), and mother's milk.
- Provides immunity in body fluids and transferred from mother to infant through breastfeeding.
- Protects mucosal surfaces by neutralizing pathogens.
IgM Antibodies
- Exists as a pentamer (mainly) or monomer; first antibody produced in primary immune response.
- Effective at activating complement system and causing agglutination in mismatched blood transfusions.
- Large pentamer allows binding to 10 antigen sites; can serve as a B cell receptor in monomeric form.
IgE Antibodies
- Monomeric, found in respiratory/urogenital mucosa, lamina propria, and lymphatic tissues.
- Involved in allergic reactions (type 1 hypersensitivity/anaphylaxis) by binding mast cells and triggering histamine release.
- Important for defense against parasites via activation of eosinophils.
- Small amounts found in blood plasma.
IgD Antibodies
- Monomeric, primarily acts as a B cell receptor on the B cell surface.
- Not typically secreted; helps B cell activation.
Immune Response & Somatic Hypermutation
- Primary immune response: first exposure—IgM produced first, then IgG.
- Secondary immune response: rapid, high IgG production due to somatic hypermutation (antibody class switching).
- Somatic hypermutation: process allowing B cells to switch from making IgM to IgG.
Passive vs. Active Immunity
- Passive immunity: antibodies provided directly (e.g., IgG via placenta, IgA via milk, antivenom therapy).
- Active immunity: antibodies produced by one's own immune system (e.g., natural infection, vaccination).
Antibody Structure
- Antibodies have two heavy chains and two light chains (constant and variable regions).
- Disulfide bonds link chains; variable regions determine antigen specificity.
- Each antibody molecule has two antigen-binding sites; IgM pentamer has ten.
Key Terms & Definitions
- Plasma cells — White blood cells that produce antibodies.
- Monomer/Dimer/Pentamer — Forms of antibody structure; one, two, or five units, respectively.
- Complement system — A group of proteins that enhance immune responses.
- Opsonization — Marking pathogens for phagocytosis.
- Agglutination — Clumping of cells (e.g., in transfusion reactions).
- Somatic hypermutation — Genetic change in B cells to switch antibody types.
- Passive/Active immunity — Immunity obtained without/with immune system’s own response.
Action Items / Next Steps
- Review the structure and functions of each antibody class.
- Understand differences between passive and active immunity.
- Study antibody structure diagrams and variable vs. constant regions.
- Prepare for related exam questions on immune response phases and antibody functions.