CD19 is functionally and physically associated with surface immunoglobulin.

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Abstract
The pan-B and B cell-specific sIg and CD19 antigens are functionally and physically associated in the presence of anti-Ig mAb. Incubation of B cells with anti-Ig antibodies causes rapid, specific, reversible, concentration-dependent, and unidirectional comodulation of CD19 on every mature B cells studied. Comodulation is produced by mAbs specific for the .gamma., .mu., .kappa., and .lambda. chains, of Ig, and by al least one idiotype-specific mAb. Comodulation is observed using 15 CD19-specific mAbs that detect at least three different CD19 epitopes. Of 18 surface antigens studied, only CD19 is comodulated. Loss of sIg and CD19 occurs concurrently during anti-Ig modulation and demonstrates a comparable dependence on anti-Ig concentration, suggesting that these are parallel rather than serial events. Incubation with anti-Ig specifically cocaps and suggests internalization of ani-CD19 mAb. Comdulation of sIg and CD19 by anti-Ig but not anti-CD19 mAbs suggests that ligand binding enables sIg to then interact with CD19. We propose that CD19 is a component of the B cell antigen receptor and suggest that it could facilitate signal transduction by sIg-antigen complexes.

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