Page 383 - Veterinary Immunology, 10th Edition

B cells can function as antigen-presenting cells and express MHC
VetBooks.ir class II molecules on their surface. In contrast, T cell expression of

MHC class II varies between species. Both types of lymphocytes
express MHC class Ia and class Ib molecules.

WC1

Lymphocytes of the major domestic mammals express several cell
surface proteins not found in either humans or mice. The best
defined of these belong to the WC1 family. WC1 are single-chain
type I glycoproteins of 220 kDa belonging to the “scavenger

receptor cysteine-rich” (SRCR) protein superfamily. They are
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expressed exclusively on γ/δ T cells. These WC1 T cells are found
in high numbers in the skin and mucous membranes as well as in

hemal nodes and the thymus. About 13 WC1 gene family members
occur in cattle, whereas 50 to 100 are found in sheep. Three subsets
have been defined by serological testing. Homologs of WC1 have
been identified in pigs, camels, llamas, deer, elk, platypuses, and
chicken. Although their natural ligand is unknown, WC1 proteins

probably bind to macrophages and dendritic cells and act as co-
stimulatory molecules. Some members of the family may bind
directly to bacteria such as Leptospira and Anaplasma.

Changes in Phenotype

Lymphocytes do not express the same phenotype at all stages in
their life cycle. A cell's phenotype depends on its maturity and
activation status. For example, immature human T cells carry both

CD9 and CD10. As the T cells mature within the thymus, CD9 is
lost, and the cells gain CD4 and CD8. Mature thymocytes can then
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split into two subpopulations; one population becomes CD4 , the
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other becomes CD8 . The phenotype of lymphocytes also changes
after exposure to antigen. For example, naïve T cells express high
levels of CD45R and L-selectin and low levels of CD44. Memory T
cells show the reverse of this: low levels of CD45R and L-selectin
and high levels of CD44.

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