Cluster of differentiation antigen 4 /CD4, the T lymphocyte antigen receptor component and human immunodeficiency virus coreceptor, is down-modulated when cells are activated by antigen or phorbolesters. Cluster of differentiation 4 is a glycoprotein found on the surface of cells such as T cells, monocytes, macrophages, and dendritic cells.
CD4 is a co-receptor that assists the T cell receptor (TCR). Using its intracellular domain, CD4 amplifies the signal generated by the TCR by recruiting an enzyme, the signaling cascade of an activated T cell. Various types of T helper cells are thereby produced. CD4 also interacts directly with MHC class II molecules on the surface of the antigen-presenting cell using its extracellular domain. During antigen presentation, both the TCR complex and CD4 are recruited to bind to different regions of the MHCII molecule.
HIV infection leads to a progressive reduction in the number of T cells expressing CD4. Medical professionals refer to the CD4 count to decide when to begin treatment during HIV infection. Normal blood values are usually expressed as the number of cells per microliter of blood, with normal values for CD4 cells being 500-1200 cells/mm3. A CD4 count measures the number of T cells expressing CD4.
CD4 continues to be expressed in most neoplasms derived from T helper cells. It is therefore possible to use CD4 immunohistochemistry on tissue biopsy samples to identify most forms of peripheral T cell lymphoma and related malignant conditions. The antigen has also been associated with a number of autoimmune diseases such as vitiligo and type I diabetes mellitus.
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