Any red cell antibody that binds its target antigen best at levels below body temperature (37 C) is commonly referred to as a “cold antibody” (this, of course, contrasts to “warm” antibodies that react best at or near body temperature). Some prefer the term “room temperature” antibodies, since not all of these actually react best in the “cold” (4 C, for example). Most cold antibodies are not clinically significant (i.e., they don’t cause hemolytic transfusion reactions or hemolytic disease of the fetus/newborn. Most cold antibodies are of the IgM type, which explains why they don’t cause HDFN (IgM doesn’t cross the placenta). Anti-Lea, -Leb, -M, -N, and -P are common cold antibodies. The one VERY major exception to the above comments about clinical insignificance is found in the ABO system. ABO antibodies are cold reactive in that they react BEST below body temperature, but are obviously very significant and capable of causing damage to adult and fetal red blood cells.
One common and sometimes important type of cold antibody is a cold autoantibody.
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