Page 432 - Veterinary Immunology, 10th Edition

VetBooks.ir B Cell Antigen Receptors

Each B cell is covered with about 200,000 to 500,000 identical
antigen receptors (BCRs), many more than the 30,000 antigen

receptors (TCRs) expressed on each T cell. Each BCR is constructed
from multiple peptide chains and, like the TCR, can be divided into
antigen-binding and signaling components. Unlike the TCR,
however, the BCR can also bind antigens when in solution.
Antibodies are simply soluble BCRs released into body fluids; they

all belong to the family of proteins called immunoglobulins
(Chapter 14).

Antigen-Binding Component

The antigen-binding component of the BCR (or immunoglobulin) is
a glycoprotein of 160 to 180 kDa consisting of four linked peptide
chains. These chains consist of two identical pairs: two heavy
chains, each 60 kDa in size; and two light chains, about 25 kDa each

(Fig. 15.1). The light chains are linked by disulfide bonds to the
heavy chains so that the complete molecule forms the letter Y. The
tail of the Y (called the Fc region) is formed from paired heavy
chains and attaches to the B cell surface. The arms of the Y (called

the Fab regions) are formed by paired light and heavy chains, and
they bind antigens (Fig. 15.2). The antigen-binding sites are formed
by the grooves between the light and heavy chains. Thus each BCR
has two identical antigen-binding sites.

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