CD molecules are cell surface markers that appear or disappear when cells (leukocytes, red blood cells, platelets, and vascular endothelial cells, etc.) differentiate or become different lineages, different segments of cells, become active or diseased. Most CD molecules are transmembrane proteins or glycoproteins, including extracellular regions, transmembrane regions, and cytoplasmic regions. Some CD molecules are "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.