Sera

1. an amber-coloured, protein-rich liquid which separates out when blood coagulates.

   the researchers examined blood serum obtained from 471 men and women

   sera were stored at −20°C until tested

2. the blood serum of an animal used to provide immunity to a pathogen or toxin by inoculation or as a diagnostic agent.

   doctors treated the bites of carpet vipers with serum produced from local snakes

late 17th century: from Latin, literally ‘whey’.