CD stands for cluster of differentiation, the number that follows is arbitrarily assigned. In the full designation the cell type and nature and molecular weight of the antigen are given in brackets, for CD4, this is as follows: [T,gp55]. CD4 is present on a subset of T lymphocytes(helper/inducer T cells) and is also expressed at a lower level on monocytes, tissue macrophages and granulocytes. The antigen is involved in binding to MHC class II molecules. The intracellular domain of the antigen is associated with p56lck protein tyrosine kinase.
