Page 377 - Veterinary Immunology, 10th Edition

from an immunological viewpoint, the most important structures
VetBooks.ir are the receptors they use to recognize antigens. These are

abbreviated TCR (T cell antigen receptor) or BCR (B cell antigen
receptor). Both the TCR and BCR are complex structures assembled

from many different protein chains. Some of these protein chains
bind antigen molecules, whereas others are used for signal
transduction. Additional complexity occurs in lymphocyte
subpopulations. For example, there are two populations of T cells

differentiated by their TCR antigen-binding chains. One uses paired
α and β peptide chains (TCR α/β) and the other uses paired γ and δ
chains (TCR γ/δ) (Chapter 14). Subpopulations of B cells use one of
five different heavy chains (γ, µ, α, ε, and δ) in their BCRs. BCRs

also differ from TCRs in that they are shed from the B cell in large
amounts into tissue fluid and the blood, where they are called
antibodies. Thus antibodies are simply soluble BCRs (Chapter 15).
Innate lymphoid cells do not have variable antigen receptors like

T and B cells. They have germline encoded receptors that can bind
to molecules expressed on healthy normal cells but not on diseased,
abnormal cells. Innate cells are turned off when they bind to these
target molecules on normal cells. On the other hand, innate cells

will kill target cells that fail to express major histocompatibility
complex (MHC) molecules (Chapter 19).
CD3 is the collective designation given to the set of signal
transducing proteins in the TCR. CD3 is therefore found on all T

cells. Another protein, CD4, is found only on helper T cells. CD4
molecules are receptors for MHC class II molecules on antigen-
presenting cells. A third protein, CD8, is, in contrast, only expressed
on T cells that attack and kill abnormal cells, the cytotoxic T cells.

CD8 molecules are receptors for MHC class I molecules. Most
human and mouse T cells express either CD4 or CD8, but rarely
−
+
both. For example, about 65% of human T cells are CD4 , CD8 , and
−
+
−
30% are CD4 , CD8 . The remaining T cells express neither (CD4 ,
−
+
CD8 ) and are said to be double negative. The ratio of CD4 to CD8 +
cells in blood may be used to estimate lymphocyte function. An
elevated CD4 count implies increased lymphocyte reactivity
because helper cells predominate, whereas a high CD8 count
implies depressed lymphocyte reactivity. The relative proportions

of CD4 and CD8 cells differ between humans and other mammals

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