CD53 is a 35-45 kD type III tetraspan membrane protein. It is expressed on all leukocytes and a subset of thymocytes, but not on platelets, erythrocytes, or non-hematopoietic cells. CD53 has been proposed to be involved in signal transduction and thymopoiesis. It has been shown to interact with a number of proteins, including IL-4, CD20, CD2, CD9, IL-2, VLA-4, and CD4. Cross-linking of CD53 promotes B cell activation. The CD53 antigen is highly glycosylated and treatment with endoglycosidase F reduces the apparent molecular weight of this protein by approximately 25kD.