An antibody produced by one individual that reacts against antigens from a genetically different individual of the same species, important in blood transfusions and organ transplants.
From Greek 'allo-' meaning 'other' or 'different,' combined with 'antibody' (anti- + body). The prefix emerged in medical terminology in the late 19th century to describe immune reactions between different individuals.
Your immune system can actually attack a blood transfusion from another person because it recognizes their cells as 'foreign'—alloantibodies are like your body's border patrol, which is why blood type matching is critical!
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