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Clusters of differentiationComments

Clusters of differentiation

CDOverview
CD1Human CD1 is encoded by five non-polymorphic and closely linked (very near each other) genes on Chromosome 1. These genes -- CdD1a,b,c,d,e -- have an intron/exon structure similar to MHC Class I genes and encode proteins homologous (similar) to MHC Class I and MHC Class II proteins. However, CD1 proteins are able to present non-peptide antigens to T cells, including mycobacterial cell wall lipids & glycolipids with hydrophobic lipid tails and hydrophilic heads. CD1 has a very narrow and hydrophobic binding pocket, thus suggesting that the lipid actually nestles within the pocket with just its hydrophilic head exposed. T cells specific for that hydrophilic end then bind the CD1-antigen complex, similar to T cell binding of the peptide-MHC complex.
CD3Present on all T cells. Necessary for TCR signal transduction and surface presentation. CD3 is a multicomponent signal-transducing complex accompanying the T cell receptor with function similar to the Ig-α/Ig-β B cell receptor complex. It is a complex of five invariant polypeptide chains which associate to form three dimers: a γε (gamma-epsilon) heterodimer; a δε (delta-epsilon) heterodimer; and a ζζ (zeta-zeta) homodimer or a ζη (zeta-eta) heterodimer. The ζ and η chains are encoded by the same gene, but differ at their carboxyl-terminal ends due to alternative RNA splicing.

Part of the immunoglobulin superfamily, the γ, δ and ε chains contain extracellular, transmembrane and cytoplasmic domains. The ζ chain is distinct, with shorter extracellular and longer cytoplasmic domains. All CD3 chains contain a negatively charged amino acid (aspartic or glutamic acid) which interacts with one or two positively charged transmembrane TCR residues.

CD3 chains have an immunoreceptor tyrosine-based activation motif (ITAM) located in their cytoplasmic tail. Also found on the Ig-α/Ig-β heterodimer of the B cell receptor complex and IgE and IgG F(c) receptors, ITAMs interact with tyrosine kinases and are critical for signal transduction. In CD3, the γ, δ and ε chains each contain a single ITAM, while ζ and η chains contain three ITAMs.
CD4CD4 binds to Class II MHC molecules, and is a 55kD monomeric membrane glycoprotein containing: four extracellular immunoglobulin-like domains (D1, D2, D3 and D4); a hydrophobic transmembrane region; and a long cytoplasmic tail containing three serine residues that can be phosphorylated. Like CD8, the extracellular domain of CD4 binds to the conserved regions of MHC molecules on antigen-presenting cells. The membrane-distal domain (furthest from the membrane) of CD4 binds Class II MHC molecules at a hydrophobic pocket formed by the α2 and β2 domains.
CD5CD5 is a marker typically found on T cells, but also present on B-1 cells (not B-2 cells, aka conventional B cells).
CD8CD8 binds to Class I MHC molecules, and is usually a disulfide-linked αβ heterodimer or αα homodimer. The α and β chains are both 30-38kD glycoproteins with: an extracellular immunoglobulin-like domain; a stalk; a hydrophobic transmembrane region; and a cytoplasmic tail of 25 to 27 residues, several of which can be phosphorylated. Like CD4, the extracellular domain of CD8 binds to the conserved regions of MHC molecules on antigen-presenting cells. CD8 binds to Class I α2 and α3 domains, and interacts somewhat with β2-microglobulin. Upon binding CD8, the Class I α3 domain changes slightly; only a single CD8 can bind a Class I MHC molecule at a time.
CD11See CD18
CD16An F(c)γ receptor, CD16 stimulates binding and uptake of antigens for antigen presentation.
CD18Integrins are heterodimeric, composed of a CD18 β subunit bound to a CD11a, b or c α subunit. The three integrins are: LFA-1 (CD18/CD11a), essential for adherence of T cells to APCs for T cell activation; macrophage-1 antigen (CD18/CD11b), aka integrin αMβ2, a receptor of complement byproducts on macrophages; and integrin αxβ2 (CD18/CD11c), also a complement receptor.
CD19Part of B cell coreceptor, with a long cytoplasmic tail with docking sites. Other components of the B cell coreceptor are CD21 and CD81.
CD21Aka CR2, CD21 is part of B-cell coreceptor and binds C3d, a byproduct of complement. Since B cells are part of acquired immunity and complement is part of innate immunity, this is an example of different branches of the immune system interacting together. Other components of the B cell coreceptor are CD19 and CD81.
CD22Present on the membrane of resting B cells, CD22 delivers a negative signal that makes activation of B cells more difficult.
CD23CD23, aka F(c)εRII, binds Ige.
CD24A molecule known as heat stable antigen (HSA).
CD25The α chain of the IL-2 receptor, present on pre-B cells.
CD28A co-receptor of the T cell receptor. Important for T cell activation.
CD32An F(c)γ receptor, CD16 stimulates binding and uptake of antigens for antigen presentation.
CD34Present on about 1% of hematopoietic stem cells. Not unique to stem cells, but irradiated mice (completely lacking stem cells) inoculated with an enriched population of CD34+ cells can restore hematopoiesis.
CD40A molecule on the surface of B cells which binds CD154 (aka CD40L) on the TH cell surface. CD40 is involved in the formation of a T-B conjugate. Also, CD40 is a tumor necrosis factor -- a family of cell surface proteins and cytokines which regulate cell proliferation and apoptosis. Its ligand, CD40L (CD154) belongs to the TNF receptor (TNFR) family. CD40 binding to T cell CD40L is necessary for immunoglobulin gene rearrangement for a functional antibody.
CD40LSee CD154
CD43Leukosialin. Only expressed on pro-B cells.
CD45RAka B220, CD45R is a protein tyrosine phosphatase found on leukocytes. As a marker unique to B cells, B220+ cells are usually assumed to be B cells.
CD64An F(c)γ receptor, CD16 stimulates binding and uptake of antigens for antigen presentation.
CD80Also known as B7-1, CD80 is a principal costimulatory molecule present on antigen presenting cells.
CD81Also known as TAPA-1, CD81 is part of the B cell coreceptor. Other components of the B cell coreceptor are CD19 and CD21.
CD86Also known as B7-2, CD86 is a principal costimulatory molecule present on antigen presenting cells.
CD152Also known as CTLA-4.
CD154Expressed on the activated TH cell membrane, CD154 (aka CD40L) is a tumor necrosis factor receptor (TNFR) protein which binds CD40 (a tumor necrosis factor) on the surface of B cells. Involved in the formation of a T-B conjugate. Toger with AID, induces isotype switching. B cell CD40 binding to T cell CD40L is necessary for immunoglobulin gene rearrangement for a functional antibody.