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Clusters of differentiationComments
parentImmunology
siblingsAntibodiesB cellB Cell ReceptorCells of the immune systemClonal Expansion TheoryHematopoiesisHumoral responseImmunological disordersImmunological PathwaysImmunology knockoutsInstructional theory vs Selective theoryNatural Killer CellsSCIDT cellT Cell Receptor (TCR)

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.