1. A substance in blood serum that destroys bacteria and other microorganisms; a complement protein.
Usage: technical; immunology; largely historical term; modern usage favors 'complement'

Examples

• Early immunologists discovered that alexin in blood serum could kill invading bacteria.
• The alexin present in normal serum was found to be heat-sensitive.
• Researchers studied how alexin contributed to the body's natural defense mechanisms.
• The term alexin has been largely replaced by the more precise term complement in modern immunology.