Naturally occurring anti-P1 with high thermal amplitude complicating ABO blood grouping.

Anti-P1 is immunoglobulin M type and occurs naturally; it is often detected as a weak- and cold-reactive antibody. However, in rare cases, it is reactive at 37°C or shows hemolytic transfusion reactions. The presence of high thermal amplitude of anti-P1 cannot be ignored and requires cross-match compatible red blood cells for transfusion.