VAP-1 functions via 2 steps during PMN transmigration. First, an antibody-dependent epitope is engaged to allow initial contacts between the PMNs and VAP-1. Then, an oxidative reaction takes place that ensures transmigration of the PMNs. If either step is blocked, the transmigration is halted. The PMN counter-receptor for the antibody-dependent epitope of VAP-1 and the leukocytic substrate for VAP-1 may be the same molecule or 2 separate molecules. Anti-VAP-1 mAbs do not interfere with entry of small model substrates or substrates expressed on the leukocyte surface into the enzymatically active site of VAP-1. Substr indicates substrate; Pretr, pretreated; and Transmigr, transmigration.