Editing CD4

[[file:PDB_1wip_EBI.jpg|thumb|PDB 1wip EBI]]
 [[file:CD4_correceptor.png|thumb|CD4 correceptor|left]] 
[[file:CD4_receptor.png|thumb|CD4 receptor]] 
[[file:CD4_and_LAG-3_with_opposing_functions.png|thumb|CD4 and LAG-3 with opposing functions|left]] 
.[[file:Reference_ranges_for_blood_tests_-_white_blood_cells.png|thumb|Reference ranges for blood tests - white blood cells]] 
{{Infobox protein
| name = CD4
| image = 
| caption = 
| width = 
| symbol = CD4
| altsymbols = 
| pdb = 
| refseq = NM_000616
| uniprot = P01730
| chromosomal_location = 12p13.31
}}

'''CD4''' (cluster of differentiation 4) is a [[glycoprotein]] found on the surface of [[immune cells]] such as [[T helper cells]], [[monocytes]], [[macrophages]], and [[dendritic cells]]. It is a co-receptor that assists the [[T cell receptor]] (TCR) in communicating with an [[antigen-presenting cell]].

==Structure==
CD4 is a single-chain transmembrane [[glycoprotein]] with four extracellular [[immunoglobulin]]-like domains (D1 to D4) that are encoded by the [[CD4 gene]] located on [[chromosome 12]] in humans. The D1 domain is responsible for binding to the [[major histocompatibility complex]] (MHC) class II molecules on [[antigen-presenting cells]].

==Function==
CD4 plays a critical role in the immune system by aiding the activation of [[T helper cells]]. When the TCR on a T helper cell recognizes an antigen presented by an MHC class II molecule, CD4 binds to the MHC class II molecule, stabilizing the interaction and enhancing the signal transduction that leads to T cell activation. This process is essential for the adaptive immune response, including the activation of [[B cells]] to secrete [[antibodies]] and the activation of [[cytotoxic T cells]] to kill infected cells.

==Clinical Significance==
CD4 is a primary receptor for the [[human immunodeficiency virus]] (HIV). HIV binds to CD4 on the surface of T helper cells, allowing the virus to enter and infect the cell. The progressive loss of CD4+ T cells in HIV-infected individuals leads to [[acquired immunodeficiency syndrome]] (AIDS), characterized by a severely weakened immune system.

==Measurement==
The number of CD4+ T cells in the blood is a key indicator of immune function in individuals with HIV. CD4 counts are used to monitor the progression of HIV infection and to guide treatment decisions. A normal CD4 count ranges from 500 to 1,500 cells per cubic millimeter of blood. A count below 200 cells/mm³ is one of the criteria for diagnosing AIDS.

==Related Pages==
* [[T helper cell]]
* [[Major histocompatibility complex]]
* [[Antigen-presenting cell]]
* [[Human immunodeficiency virus]]
* [[Acquired immunodeficiency syndrome]]
* [[B cell]]
* [[Cytotoxic T cell]]

[[Category:Immunology]]
[[Category:Proteins]]
[[Category:Transmembrane proteins]]
[[Category:Human proteins]]

{{Immunology-stub}}