Page 373 - Veterinary Immunology, 10th Edition

VetBooks.ir Lymphocyte Surface Molecules

All cells express thousands of different protein molecules on their
surface. Hundreds of these surface molecules have been

characterized, especially on human and mouse lymphocytes (Box
13.1). As each molecule is characterized it is usually given a
functional or chemical name. It is also usually given a cluster of
differentiation (CD) designation as well (Figs. 13.7 and 13.8).
Currently, the CD system assigns sequential numbers to each

molecule: CD1, CD2, CD3, and so on, up to CD371. Since arbitrary
numbers are difficult to remember, the basic principle used in this
text is that if the molecule's common name is well accepted or

describes its function, that name will be used. Examples include the
IgA receptor, FcαR (CD89), the interleukin-6 receptor (CD126), and
L-selectin (CD62L). CD nomenclature is also used for molecules for
which the designation is well accepted, such as CD8 and CD4. A list
of the most relevant CD molecules and their functions can be found

in Appendix 1.

Box 13.1

A Note on Cell Phenotypes

All the cells of the body arise from a single precursor cell, the
fertilized ovum. As the embryo develops and grows, cells
differentiate both structurally and biochemically. They do this by
activating required genes while turning off unneeded ones. One

obvious result is that cells acquire a characteristic morphology.
Histologic examination shows these structural differences and has
provided much useful guidance regarding a cell's function.
Structural differences are limited, however, in what they can tell

us. For example, T and B cells look identical but differ significantly
in their biochemistry and their function. As a result, biochemical
differences must be determined to identify functional cell types.
One of the best ways to do this is to examine the proteins

expressed on the cell surface. Cells express hundreds of different
proteins on their surface, and their identification provides a
powerful tool to characterize cells. The CD system of identifying

373