Immunophenotyping is the analysis of heterogeneous populations of cells for the purpose of identifying the presence and proportions of the various populations of interest. Antibodies are used to identify cells by detecting specific antigens expressed by these cells, which are known as markers. These markers are usually functional membrane proteins involved in cell communication, adhesion, or metabolism.
Clusters of Differentiation (CD) antigens are widely used for immunophenotyping. CD antigensare a series of surface proteins on leukocytes that serve to differentiate the many types of white blood cells. CD proteins serve as receptors and ligands such as CD4 and CD8 on T lymphocytes which serve to regulate the adaptive immune response. Some CD proteins regulate cell signaling, while others ensure cell adhesion which is an important aspect of adaptive immunity.
This is very confusing and is one reason why hematopathologists keep lots of aspirin in their desks. Nonetheless, if you study lymphomas, you will have to know at least the basics of this scheme. Here are the basics:
All lymphoid cells are reactive for CD45.
Almost all of B-cells are reactive for CD19, CD20 and CD22. Certain low-grade B-cell lymphomas are reactive for two markers otherwise usually found on T-cells: CD5 and CD43. Follicular center cell lymphomas (as well as very different fish, lymphoblastic lymphomas) are frequently CD10(+).
Pan T-cell markers (present on almost all T-cells) include CD2, CD3, CD5, and CD7. Most T-cells mark with either CD4 (helper cells) or CD8 (suppressor cells or cytotoxic cells).
NK cells are frequently associated with CD16, CD56, or CD57.
The extent of use of CD antigen expression (immunophenotyping) for identification of leukemias has been constrained by the technique used, flow cytometry, which commonly specifies only three CD antigens in any one assay.
CD4 immunophenotyping in HIV