PRINCIPLES OF TOXICOLOGY

10.2 BIOLOGY OF THE IMMUNE RESPONSE 193
Figure 10.1 Light and Heavy Chain Structure of IgG. IgG illustrates the basic structure of antibody proteins,
which consists of two long, heavy chains and two shorter, light chains held together by disulfide bonds. Composition
of C domains is relatively constant, while V domain varies, creating the binding specificity characteristic of
antibodies.
very early in an immune response. IgM is much larger than the other Igs, consisting of five sets of
heavy/light-chain pairs bound together at a single point with another peptide (the J chain). Its
molecular weight is about 970,000. IgA may exist as a monomer (one basic unit of two pairs of H and
L chains) or as a dimer—two basic units bound together with a J chain. The monomeric IgA has a
molecular weight of about 160,000 and is the predominant form of IgA found in serum. IgA is the
primary Ig found in secretions (e.g., tears and saliva), mostly in the dimeric form with a molecular
weight of 385,000. IgD has a molecular weight of about 184,000, and is present in very low
concentrations in serum. Its function is unclear, but it may play a role in B-cell differentiation. IgE is
slightly larger than IgG (molecular weight of 188,000), and is normally present in low concentrations
in serum. It can attach itself to leukocytes and mast cells, and is the primary antibody involved in
hypersensitivity reactions.
In cell-mediated immunity, cells carrying the antigen on their surface are attacked directly by
cytotoxic T cells (T C ) or other cell types such as natural-killer (NK) cells. In the case of T C cells,
recognition of cells to be destroyed is through interaction between processed antigen in conjunction
with MHC class I molecules on the target cell surface and an antigen receptor on the T C . In order to
be active, the T C must also receive stimulation from CD4 + cells, principally in the form of IL2.
Mechanisms of target cell recognition by NK cells are not well understood.