Cluster of differentiation, or CD molecules, are cell surface markers that are used for identification of cell types in pathology and other bioscience disciplines. The expression levels of CD markers may increase or decrease (or disappear altogether, at least to undetectable levels) when cells (for example, leukocytes, red blood cells, platelets, and vascular endothelial cells, etc.) differentiate into new and different lineages. Depending on the CD marker, the expression level may identify a phenotype for different segments of cells, such as when they become active or diseased. Most CD molecules are transmembrane proteins or glycoproteins, including extracellular regions that bind a ligand or opposing receptor, transmembrane regions to anchor the CD marker into the cell, and cytoplasmic regions that may confer some adaptor or catalytic function. Some CD molecules can also be "anchored" on the cell membrane by means of inositol phospholipids. A few CD molecules are carbohydrate haptens. The study of CD molecules can be used in many basic immunology research fields, such as the relationship between CD antigen structure and function, cell activation pathway, signal transduction and cell differentiation, etc. It can be used clinically for disease mechanism research, clinical diagnosis, disease prognosis, efficacy tracking and treatment, and more. CD molecules such as CD4, CD8, CD25, etc. can be used to identify populations of cells when studying samples by flow cytometry or immunofluorescence.
