CD5 is a 67kDa single-chain transmembrane glycoprotein expressed on mature T lymphocytes, most thymocytes and B lymphocytes subset (B-1a lymphocytes) and on many T-cell leukemias and lymphomas. It is a type I membrane glycoprotein whose extracellular region contains three scavenger receptor cysteine-rich (SRCR) domains. The CD5 is a signal transducing molecule whose cytoplasmic tail is devoid of any intrinsic catalytic activity. CD5 modulates signaling through the antigen-specific receptor complex (TCR and BCR). CD5 may serve as a dual receptor, giving either stimulatory or inhibitory signals depending both on the cell type and development stage. In thymocytes and B1a cells seems to provide inhibitory signals, in peripheral mature T lymhocytes it acts as a costimulatory signal receptor. CD19 is a member of the immunoglobulin superfamily and has two Ig like domains. The CD19 molecule is expressed on 100% of the peripheral B cells as defined by expression of kappa or lamda light chains. It is expressed on approximately 10% of normal human peripheral blood cells and approximately 60% of splenic lymphocytes. It is not expressed on granulocytes, monocytes or T cells as defined by CD3 expression. CD19 defines a pan B antigen which is expressed from the earliest stages of B progenitor development, but is lost on terminal differentiation to plasma cells. It may also be present on some early myeloid progenitors, particularly those of the monoblastic type. The CD19 antigen is expressed on approximately 12% of peripheral blood lymphocytes. It appears to be expressed on myeloid leukemia cells, particularly those of monocytic lineage. Leukemia phenotype studies have demonstrated that the earliest and broadest B cell restricted antigen is the CD19 antigen.