Antibody Structure and Function
Structural Features of IgG
- 2 L-chain domains; 4 H-chain domains
- Inter and intrachain disulfide bonds hold IgG together
- Single carbohydrate moiety in CH2 domain (plays role in transport/secretion)
Biologic Properties of IgG (8)
1. Agglutination & Precipitate Formation
2. Placental Passage: selectively passed through placenta (except IgG2) to provide passive immunity to fetus; transfer mediate by Fc region of IgG
3. Opsonizing Agent: prepares foreign matter for macrophages
4. AB-Dependent Cell-Mediated Cytotoxicity: IgG binds tumor cell, allowing NK cell to bind to Fc portion of it and kill the tumor cell
5. Complement Activation: IgG AB activate this cascade via opsonins and chemotactic
6. Toxin Neutralization: immobilizes antigen by forming an antigen-AB complex, which can be easily phagocytized
7. Bacterial Immobilization: IgG (and IgM) disable flagella of motile bacteria
8. Viral Neutralization: Bind to cellular receptor preventing initial binding of virus and facilitating phagocytosis
Structural Features of IgM
- High molecular weight molecule with a star shape (crab when antigen’s bound)
- Has a J chain for stabilization
- Hinge region is not nearly as flexible as in IgG
- CH3 domain has complement binding site
- 5 carbohydrate moieties
Biological Properties of IgM (3)
First isotype to appear in serum after exposure to antigen/vaccination
- Agglutination
- Isohemagglutinins: Natural AB reactive with RBC antigens of ABO series; these AB have carb epitopes similar to antigens of A or B blood group
- Complement Activation: Most effective molecule at this
Structural Features of IgA
- Found in sero-mucous secretions as a dimer stabilized against proteolysis by a combination with a local secretory component
- Large # of disulfide bonding including intrachain btw CH2 and CH3 and J chain binding
- Two subclasses: IgA1 (lots in serum) and IgA2 (lots in extravascular secretions)
Biological Properties of IgA (3)
1. First-line barrier of protection for mucosal services
2. Bactericidal for gram-negative organisms
3. Viricial agent preventing attachment to cell surfaces
Structural Features of IgD
- In trace amounts in serum
- Is NOT synthesized/secreted by mature plasma cells
- Hinge region is the largest of any Ig hinge allowing for max flexibility
Biological Properties of IgD (1)
1. Found on surface of mature B cells and helps them mature
Structural Features of IgE
- In trace amounts in serum
- Does not have a hinge region
- Fc portion has recognition site on mast cell surface and can cause release of mast cell contents (histamine)
- Large number of carb moieties
Biological Properties of IgE
Responsible for symptoms of hay fever and extrinsic asthma
1. Type 1 Hypersensitivity: IgE response against innocuous antigens producing release of pharmacological mediators that cause an acute inflammatory reaction; if antigen is introduced IV (i.e. bee sting) is can cause systemic sensitization leading to cardiac arrest
2. Mast Cell Content Release: Arrival of an antigen causes (a) cross-linking of the receptors, (b) breakdown of PI to inositol triphosphate, (c) generation of DAG, and (d) increase in free calcium
3. Parasitic Infections: Elevated IgE levels are seen as well as same mast cell content release, which can have bronchoconstrictive/vasodilatory effects
Tags: Agglutination, Bacterial Immobilization, Complement Activation, ero-mucous secretions, extrinsic asthma, Hypersensitivity, Isohemagglutinins, Mediated Cytotoxicity, Opsonizing Agent, Parasitic Infections, Placental Passage, serum, Structural Features of IgG, Viral Neutralization
