What does the term 'reagin' refer to in immunology?

Reagin refers specifically to an antibody-like substance that is produced in response to certain infections, most notably in the context of syphilis. The term is often associated with non-treponemal tests for syphilis, such as the Rapid Plasma Reagin (RPR) test and the Venereal Disease Research Laboratory (VDRL) test. These tests detect the presence of reagin, which is made up of immunoglobulin M (IgM) antibodies, as a response to the lipid antigens released from damaged host cells and certain Treponema bacteria.

This distinguishing feature of reagin is crucial in immunology and clinical diagnostics, as it highlights the body's immune response to specific infections rather than to foreign invaders in general or standard antibodies traditionally involved in infection. Understanding reagin's role becomes particularly important in identifying infections and monitoring disease progression or treatment responses.