CD5 antigen is a transmembrane glycoprotein, which is expressed on the surface of almost all mature human T cells (about 10% of CD4 + T cells are CD5 -). In immature (CD34 +) T cells, the expression of CD5 is weak, and the expression intensity increases with maturation. CD5 is also expressed in a small number of normal human B cells (20% of B cells were in peripheral blood, scattered in the mantle region of lymph nodes).