HLA class II antibodies bind to and induce activation of matched monocytes. (A) Anti-DR52 (top) and anti-DR7 (bottom) were incubated with matched or unmatched monocytes as indicated. FITC-labeled anti-IgG or -IgM was used as a secondary antibody (black histograms). Gray histograms indicate results obtained with control plasma. (B) Matched (black bars) and unmatched (white bars) human monocytes were incubated with human anti-DR52 or anti-DR7 as indicated. Supernatants were investigated for the presence of cytokines and LTB₄ after 6 hours, as indicated. P < .01 between matched and unmatched experiments for all groups. < DL indicates below detection limit. Concentrations are given as mean values + SDs.