Cold Agglutinins

Cold agglutinins are clinically relevant antibodies that are implicated in intravascular hemolysis. At normal body temperature, these antibodies do not cause hemolysis. However, when RBCs get out into the periphery and temperatures drop, the antibodies can attach to red blood cells, causing hemolysis primarily through the complement system. They are called cold agglutinins because when they are placed in cold temperatures in a laboratory setting, the antibodies will attach to RBCs and cause agglutination.

Remember the 3 M's of cold Agglutinins:
IgM
Mycoplasma pneumoniae (can trigger cold agglutinins)
Mononucleosis (EBV, CMV infection)

Warm agglutinins are caused by IgG. To remember the difference, you can remember that
"Maine is colder than Georgia."