Fab (containing the variable/hypervariable) regions consisting of light (L) and heavy (H) chains recognizes antigens. Fc region of IgM and IgG fixes complement. Heavy chain contributes to Fc and Fab regions. Light chain contributes only to Fab region.

Fab: Fragment, antigen binding- Determines idiotype: unique antigen-binding pocket, only 1 antigenic specificity expressed per B cell

Fc: Constant - Carboxy terminal- Complement binding (C_H2)- Macrophage binding (C_H2 and C_H3)- Carbohydrate side chains- Determines isotype (IgM, IgD, etc)

Generation of antibody diversity (antigen independent)1. Random recombination of VJ (light-chain) or V(D)J (heavy-chain) genes2. Random addition of nucleotides to DNA during recombination by terminal deoxynucleotidyl transferase (TdT)3. Random combination of heavy chains with light chains.

Generation of antibody specificity (antigen dependent)4. Somatic hypermutation and affinity maturation (variable region)5. Isotype switching (constant region)