Page 436 - Veterinary Immunology, 10th Edition

The number of constant domains differs between immunoglobulin
VetBooks.ir heavy chain classes. There are three constant domains in a γ heavy

chain; they are labeled, from the N-terminal end, C 1, C 2, and
H
H
C 3. Three constant domains are also found in α and most δ chains,
H
whereas µ and ε chains have an additional constant domain called

C 4.
H
Since heavy chains are paired, the domains in each chain come
together to form structures by which antibody molecules can exert

their biological functions. Thus V and V together form an antigen-
H
L
binding site, and C 1 and C together stabilize the antigen-binding
L
H
site. The paired C 2 domains of IgG contain a site that activates the
H
classical complement pathway (Chapter 4) and a site that binds to
Fc receptors on phagocytic cells (Fig. 15.5). The heavy chain also
regulates the transfer of IgG into colostrum (Chapter 23) and
antibody-mediated cellular cytotoxicity (Chapter 18). When

immunoglobulin molecules act as BCRs, their Fc region is
embedded in the B cell surface membrane. These cell-bound
immunoglobulins differ from the secreted form in that they have a
small transmembrane domain located at their C-terminus. This

contains hydrophobic amino acids that associate with the cell-
membrane lipids.

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