Foreign antigens opsonized by complement degradation products may be bound by both the B-cell antigen receptor (BCR) and CR2-CD19 complexes. Under these circumstances, the extensive cytoplasmic tail of CD 19 endows the BCR with additional tyrosine kinase activity and with potential docking sites for molecules invoh, ed in cell signalling. Here, Carel van Noesel and colleagues argue that cooperation between BCR and CR2-CD19 at both the extra-and intracellular level provide for optimal recognition of antigen and amplification of ensuing intracellular signals.

It ha~ recently been established that the B-cell antigen receptor complex (BCR) comprises at least four different mol,: cules TM. The variable antigen-binding portion, that is, the membrane-botmd immunoglobulin (mIg) tetramer of heavy (H) and light (L) chains, adjoins a heterodimer of non-polymorphic transmembrane proteins. On human B cells, the mIgM-associated molecules, designated lg0t and lgl3, have been defined as 47 kDa and 37 kDa•,-4 glycoprotelns-, whereas their mouse counterparts have reported molecular masses of 34 kDa and 39 kDa, respectively. After removal of N-linked carbohydrates, the size of both mlgM-associated molecules, both human and mouse, is in the 20-25 kDa rangel-S.