Alloimmunization, as defined elsewhere, simply means formation of antibodies against non-self antigens. When this occurs against antigens in the “Human Leukocyte Antigen” (or HLA) system, the process is known as HLA alloimmunization. Anti-HLA antibodies are most commonly induced by multiple pregnancies, though transfusion of blood products (especially those given before the near-universal implementation of leukocyte reduction) may also be a culprit. HLA antibodies are a big deal when a patient needs a hematopoietic progenitor cell transplant, as well as in those needing solid organ transplants (though less so). In addition, developing these antibodies may make a patient resistant, or “refractory” to the effect of platelet transfusions due to the fact that HLA antibodies are present on the platelet surface.