Using EBV-transformed antigen-specific B-cell clones as APC, we have started to analyze the factors that govern the uptake of antigen by B cells and lead to its efficient presentation to T cells. The binding of antigen to SIg can be blocked by soluble antibodies that react with the same epitope that is seen by the B cell's SIg, but not by antibodies that bind to different epitopes. By studying the antigen presenting capacity of B cells that had been pulsed with antigen in different conditions, we came to the conclusion that specific B cells work as vacuum cleaners, i.e. over time they accumulate the antigen that binds to SIg. This effect results from the difference between the rate of influx (approximately 5-10 times the amount of antigen bound is internalized every hour by receptor-mediated endocytosis) and the rate of loss (the half-life of processed antigen relevant for T-cell activation is about 1 d). T cells specific for mouse Ig are triggered much more efficiently by mouse anti-human Ig than by mouse antibodies directed against other B-cell surface antigens, suggesting that SIg are extremely efficient for antigen internalization and processing.