Page 434 - Veterinary Immunology, 10th Edition

Light chains are constructed from two domains each containing
VetBooks.ir about 110 amino acids. The amino acid sequences in the C-terminal

domains in the BCRs from different B cells are identical and are
called constant domains (C ). In contrast, the sequences in the N-
L
terminal domains differ in each B cell and so form variable domains
(V ). Mammals also make two types of light chains, called κ (kappa)
L
and λ (lambda). Although their amino acid sequences are different,
they are functionally identical. The ratio of κ to λ chains in BCRs
varies among mammals, ranging from mice and rats, which have

more than 95% κ chains, to cattle and horses, which have 95% λ
chains. Primates such as the rhesus monkey and the baboon have
50% of each, whereas humans have 70% κ chains. Carnivores such
as cats and dogs have 90% λ chains.

Heavy Chains

Immunoglobulin heavy chains are constructed from four or five
domains each of about 110 amino acids. The N-terminal domain is a
variable (V ) domain. The remaining three or four domains show
H
few sequence differences and thus are constant (C ) domains.
H
Mammalian B cells make five different classes of heavy chain that

differ in their sequence and domain structure. As a result, each class
has a different biological activity. The five different
immunoglobulin heavy chains are called α, γ, δ, ε, and µ. These

heavy chains determine the immunoglobulin class (or isotype).
Thus immunoglobulin molecules that use α heavy chains are called
immunoglobulin A (IgA), and those that use γ chains are called
IgG; µ chains are used in IgM, δ chains in IgD, and ε chains in IgE.

Variable Regions

When the sequences of V domains from light and heavy chains are
examined in detail, two features become apparent. First, their
sequence variation is largely confined to three regions each
containing 6 to 10 amino acids, within the variable domain (Fig.

15.3). These regions are said to be hypervariable. Between these
three hypervariable regions are relatively constant sequences called
framework regions. The hypervariable regions on paired light and
heavy chains determine the shape of the antigen-binding site and

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