The Clusters of Differentiation (often abbreviated as CD) is a protocol used for the identification and investigation of cell surface molecules providing targets for immunophenotyping of cells. In terms of physiology, CD antigens can act in numerous ways, often acting as receptors or ligands (the molecule that activates a receptor) important to the cell. A signal cascade is usually initiated, altering the behavior of the cell. Some CD antigens do not play a role in cell signaling, but have other functions, such as cell adhesion, cell activation, and cell inhibition.
Cells interact with each other, and their substrate, throughout their lifetime. The adhesion complex that interacts with the cellular substrate, or extracellular matrix, and one that interacts with another cell. These complex cellular structures involve many proteins, such as CD antigens. Cell adhesions are mediated by either transmembrane cell-adhesion molecules, or adhesion receptors. The adaptive immune or specific immune response consists of antibody responses and cell-mediated responses, which are carried out by different lymphocyte cells, B cells and T cells, respectively.When immunologists began to study different kinds of lymphocytes, they began to classify them by the CD antigens that were on their surface, as know as CD4+ T cells are helper T cells, and CD8+ T cells are cytotoxic T cells.
Some CD antigens are cells surface proteins as receptors. The CD antigens interaction are fundamental to cell signaling. When such CD antigen activate its receptor, the signal is carried into the cell usually by means of a second messenger. CD40, a ligand for the B cell, is a key component of activation antigen of T cells. The B7 protein is present on the antigen-presenting cell surface, and it interacts with the CD28 receptor on the T cell surface. There are two major types of B7 proteins: B7-1/CD80, and B7-2/CD86.
The CD antigens are commonly used as cell markers in immunophenotyping, allowing cells to be defined based on what molecules are present on their surface. These markers are often used to associate cells with certain immune functions. While using one CD molecule to define populations is uncommon (though a few examples exist), combining markers has allowed for cell types with very specific definitions within the immune system.
CD antigens are utilized in cell sorting using various methods including flow cytometry.
Data from the articles available to manually extracted and resulted in 6153 data points containing the expression of 305 CD molecules on 206 hematopoietic cells from 1223 literature sources. Proteins with in the CD antigens were found to beinvolved in signal transduction, immune response and adhesion processes to a higher degree than other surface proteins.
Functional characterization of CD antigens, compared to all UniProtKB annotated surface proteins
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