Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
CD4 is a cell-surface glycoprotein found on the mature helper T cells and immature thymocytes, as well as on monocytes and macrophages. (Some cytotoxic T cells have CD4 protein as well.) Normally, about 65% of T cells in the blood are CD4+ (have CD4 protein protruding from their membrane).
A mature T cell with either have CD4 or CD8, but not both. (During one stage of development T cells develop CD4 and CD8 receptors, but they eventually are differentiated in the thymus and become more specialised.)
CD4 is a member of the immunoglobulin superfamily (see the picture below). You can see the external part of the protein has four "domains" in the representation, and these are the bulk of the protein. Together, these make up the first 371 amino acids in the protein (i.e. amino acid residues 1-371). Of the remaining portion, the transmembrane section is amino acids 372 through 395, and the cytoplasmic section (the bit that lies on the inside of the cell) is amino acids 396-433.
A very useful measure of the progression of HIV infection in a person is the "CD4 count", or a chemical estimation of the numbers of CD4+ T cells a person has in their body, since cells with CD4 surface protein are the ones which are targeted and eventually destroyed by HIV. Normal CD4+ counts are between 500 and 1600; but a person with HIV AIDS commonly has a CD4+ count of less than 200.