An antibody that targets antigens present on the patient or donors’ own red blood cells (in contrast to alloantibodies, which target non-self red cell antigens). Autoantibodies are fairly common in hospitalized patients, but are not usually significant (meaning, they can cause testing problems, but they don’t harm the patient). These self-directed antibodies may be categorized by the temperature at which they react best; i.e., either as “cold” autoantibodies reacting best below body temperature or as “warm” autoantibodies reacting best at or near body temperature. In general, autoantibodies will result in positive direct antiglobulin tests (though not always). Autoantibodies cause problems in blood bank compatibility testing because they tend to react against a wide array of donor red blood cells, and finding compatible blood may be very difficult.

For those on the clinical side, the most important thing to know about patients with autoantibodies is that their blood bank testing will almost always be delayed (sometimes by hours and hours), but once we do all the work we need to do, the patient can usually be safely transfused (though we may ask you some questions about the patient’s current status before transfusion, including whether there is evidence the autoantibody is destroying his or her own red cells).