1) Blood component 
An obsolete term for the bactericidal substances of cell-free serum, the activity of which is destroyed by heating at 56°C; applied by Bordet to the heat-labile substance normally present in serum and distinct from the sensitizing substance (antibody) produced by infection or immunization. In this sense it is synonymous with complement. ... Origin...
Type: Term Pronunciation: ă-lek′sin Definitions: 1. Obsolete term for serum bactericidal compenents now described within complement system.
alexin 1. A defensive substance in normal serum that destroys bacteria and other cells. 2. A name for a class of substances found in blood-serum, having the capacity of destroying bacteria, etc., and thus guarding against infection.
complement (def. 10).