Recognizes a protein of 33-55kDa, identified as CD53 (Workshop VI; Code N-L033). It is expressed on monocytes and macrophages, dendritic cells, osteoblasts and osteoclasts, and on T and B cells from every stage of differentiation but is absent from platelets, red blood cells. CD53 appears to be the marker with widest reactivity as well as the marker with the strictest specificity to hematopoietic cells. CD53 is a type III membrane with both termini in the cytoplasm and two loops in the extracellular environment. This molecule, in common with other members of tetraspan family, is involved in cellular activation as part of a signal transduction complex involving other membrane glycoproteins. CD53 crosslinking induces calcium flux on human monocyte and B cells. Cross-linking of CD53 promotes activation of resting human B-lymphocytes. This MAb recognizes CD53 transfected cells and partially inhibits T-cell proliferation induced by CD3 antibody (clone: UCHT1).