Leukocytes express distinct assortments of molecules on their cell surfaces, many of which reflect either different stages of their lineage-specific differentiation or different states of activation or inactivation. Leukocyte cell surface molecules are routinely detected with anti-leukocyte monoclonal antibodies (mAbs). Using different combinations of mAbs, it is possible to chart the cell surface immunophenotypes of different leukocyte subpopulations, including the functionally distinct mature lymphocyte subpopulations of B-cells, helper T-cells (T_H), cytotoxic T-cells (T_C), and natural killer (NK) cells (Figure 3-9, 3rd ed.).

The abbreviations used for identifying different leukocyte cell surface molecules are somewhat complex because historically three conventions have been used:

Although less intuitive, the CD antigen convention has become the accepted norm for formal designation of leukocyte surface molecules. This is a logical consequence of the fact that most leukocyte cell surface molecules were discovered only by their reactivity patterns with anti-leukocyte antibodies when little, if anything, was known about their functions. Also, because most leukocyte cell surface molecules are recognized by more than one antibody, it is easy to group antibodies according to identical reactivity patterns even though they may recognize different portions of the same molecule. An obvious problem, though, is that some antibodies recognize more than one distinct leukocyte surface structure as is the case, for example, with the structurally heterogeneous CD45 splice variants. Different variants have identical protein domains, and anti-CD45 antibodies recognizing these shared domains will cross-react with more than one CD45 variant.

A useful Table of CD Antigens has been prepared by Paul Travers [no longer found] for web viewing with well over 100 human leukocyte cell surface molecules assigned CD designations. Note that some CD antigens have also been named by one of the other conventions. For example:

• CD54 = LFA-1;
• CD80 = B7 (or now B7-1); and
• CD45 = B220.

Some CD antigens are widely expressed on different leukocyte populations while others are expressed on only one type or just a few types of leukocytes. For example:

• CD54/LFA-1 is widely expressed on a variety of hematopoietic cells.
• Conversely, CD4 is expressed almost exclusively on T helper (T_H) lymphocytes and cells of the monocyte/macrophage lineage, an important consideration for understanding the pathology of human immunodeficiency virus (HIV) which gains cell entry via the CD4 molecule.

Commercial listings of anti-CD monoclonal antibodies are easy to find on the web. For example, a fairly comprehensive listing is electronically published by Immunotech.