reagin antibody

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 serologic tests for syphilis. reaginic, adj.

in other words

n noncommittal term used for antibodies or antibody-like substances that differ in several respects from ordinary antibodies. It refers to the antibodies of allergic conditions (atopy) and to the antibody (reagin) concerned with the flocculation and complement fixation tests for syphilis.
reagin
an old term for immunoglobulin E antibody (IgE).