Immune cells possess cell surface markers designated as CD (cluster of differentiation) molecules. These cell surface proteins serve as cell surface markers for identification of the diverse immune cell types, but also have roles as cell surface receptors or ligands involved in antigen presentation, signaling and cell adhesion. There are more than three hundred cell surface markers identified in human immune cells, as well as other nonimmune cells, such as endothelial and epithelial cells, which often express these markers.
CD3, CD4 and CD8 are T-cell markers, while CD19 and CD20 are markers of B-cells. T-helper cells are generally characterized by presence of CD4 glycoprotein, and are referred to as CD4+ cells, whereas cytotoxic T-cells or killer T-cells generally express CD8 at the cell surface and are called CD8+ cells. Dendritic cells, macrophages, platelets and other hematopoietic cells express a variety of these receptors. Whether a T-cell is in the naïve or activated state can also be discriminated by presence of specific cell surface markers. Labeling cells with antibodies specific for cell surface markers is commonly used in flow cytometry to distinguish and isolate distinct immune cell populations.
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