Y W6/32 to HLA class I and then measured exocytosis by the release of VWF and the externalization of P-selectin. Antibody to HLA activates endothelial exocytosis in a dose-dependent manner over time. The biologically active complement split product, C5a, adds a slight but significant increase to antibody induction of exocytosis. Antibody to HLA alone or with C5a did not damage the cells. Cross-lin