ANTIBODY STRUCTURE AND CLASSES
Antibodies (Abs), immunoglobulin (Ig) are roughly Y-shaped molecules or combinations of such molecules. There are five major classes of Ig’s: IgG, IgA, IgD, IgE, and IgM. Table 2 summarizes the character-istics of these molecules, particularly their structure (monomer, dimer, pentamer, or hexamer), molecular weight (ranging from ~150 kDa to ~1150 kDa), functions (e.g., activate complement, FcγR binding). Among these classes, IgGs and their derivatives form the framework for the development of therapeutic antibodies. Figure 1 depicts the general structure of an IgG with its structural components as well as a conformational structure of efalizumab (anti-CD11a, Raptiva ). An IgG molecule has four peptide chains, including two identical heavy (H) chains (~50–55 kDa) and two identical light (L) chains (25 kDa), which are linked via disulfide (S-S) bonds at the hinge region. The first ~110 amino acids of both chains form the variable regions (VH and VL), and are also the antigen binding regions. Each V domain contains three short stretches of peptide with hypervariable sequences (HV1, HV2, and HV3), known as complementarity determining regions (CDRs), i.e., the region that binds the antigen. The remaining sequences of each light chain consist of a single constant domain (CL). The remainder of each heavy chain contains three constant regions (CH1, CH2, and CH3). Constant regions are responsible for effector recognition and binding. IgG can be further divided into four subclasses (IgG1, IgG2, IgG3, and IgG4). The differences among these subclasses are also summarized in Table 2.