reagin /rē″ājin/ [L, re + agere] , 1. an antibody associated with human atopy, such as asthma and hay fever. It attaches to mast cells and basophils and sensitizes the skin and other tissues. In antigen-antibody reactions it triggers the release of histamine and other mediators that cause atopic symptoms. 2. a nonspecific, nontreponemal antibody-like substance found in the serum of individuals with syphilis. It can combine with an antigen prepared as a lipid extract of normal tissue, a phenomenon that constitutes the basis of the serological tests for syphilis. See also nontreponemal antigen test. −reaginic, adj.
