An antibody or immune substance in blood serum that helps destroy bacteria and foreign organisms.
From Greek alexein (to ward off) plus -in (chemical suffix). Named in the early immunology era when scientists were discovering immune factors. Now largely replaced by 'complement' in modern immunology terminology.
Alexins were among the first substances where scientists realized your blood has tiny molecular warriors already patrolling for invaders—it was one of the foundational discoveries that proved your immune system is biochemically clever, not just physically protective.
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