1CDH, 1CDI, 1CDJ, 1CDU, 1CDY, 1G9M, 1G9N, 1GC1, 1JL4, 1Q68, 1RZJ, 1RZK, 1WIO, 1WIP, 1WIQ, 2B4C, 2JKR, 2JKT, 2KLU, 2NXY, 2NXZ, 2NY0, 2NY1, 2NY2, 2NY3, 2NY4, 2NY5, 2NY6, 2QAD, 3B71, 3CD4, 3JWD, 3JWO, 3LQA, 3O2D, 3S5L, 3T0E, 4JM2, 1WBR, 3S4S, 4H8W, 4P9H, 4Q6I, 4R2G, 4R4H, 4RQS, 3J70, 5A7X, 5A8H, 5CAY92012504ENSG00000010610ENSMUSG00000023274P01730P06332NM_001382705NM_001382706NM_001382707NM_001382714NM_013488NP_001369634NP_001369635NP_001369636NP_001369643NP_038516In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR).

[citation needed] The short cytoplasmic/intracellular tail (C) of CD4 contains a special sequence of amino acids that allow it to recruit and interact with the tyrosine kinase Lck.

The TCR complex and CD4 bind to distinct regions of the antigen-presenting MHC class II molecule.

[13][15][16] LAG-3, which is an inhibitory receptor, is upregulated in activated T cells as a kind of negative feedback loop.

HIV-1 uses CD4 to gain entry into host T-cells and achieves this through its viral envelope protein known as gp120.

they do not check the presence of viral DNA, or specific antibodies against HIV—they are used to assess the immune system of a patient.

[citation needed] National Institutes of Health guidelines recommend treatment of any HIV-positive individuals, regardless of CD4 count[20] Normal blood values are usually expressed as the number of cells per microliter (μL, or equivalently, cubic millimeter, mm3) of blood, with normal values for CD4 cells being 500–1200 cells/mm3.

[citation needed] Viral load testing provides more information about the efficacy for therapy than CD4 counts.

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.