The term CD (Cluster of Differentiation) translates to clusters or groups of differentiation, also known as leukocyte differentiation antigens. These refer to cell surface markers that appear or disappear on various lineages of white blood cells during different stages of normal differentiation, maturation, and activation processes. They are mostly glycoprotein molecules. Physiologically, CD molecules often serve as important receptors or ligands on cells and as cellular markers for immune antigen recognition. However, some CD proteins are unrelated to cell signaling, but play roles in cell adhesion, activation, or inhibition.