Anaphylactic Reactions
MECHANISM: A variation on a classic theme.
Anaphylaxis, in general, is usually caused by exposure of an antigen to a sensitized patient. The "classic" type of anaphylaxis that has been taught for years involves IgE antibodies on the surface of a mast cell targeted against an offending antigen. When the patient is exposed to the antigen a second time, the IgE stimultes the mast cells, leading to an outpouring of histamine. Histamine, in turn, leads to vasodilation, bronchospasm, skin symptoms, etc.
Anaphylactic transfusion reactions, on the other hand, do not necessarily follow the "classic" anaphylaxis pathway. Most of them do occur as a response to an antigen that they lack, most commonly Immunoglobulin A (IgA). However, exactly how the mast cell stimulation occurs is a little bit unclear. First, most people with IgA deficiency do not even have anti-IgA (the incidence of anti-IgA in deficient patients is only about 30% or so). Second, people who have anti-IgA do not usually have demonstrable IgE class antibodies targeted against IgA; rather, they typically have anti-IgA that is of the IgG or IgM class. Finally, less than 20% (in one study) of the people with IgA deficiency who actually have anaphylactic reactions have anti-IgA of any type! Obviously, something else is going on other than a classic pathway to anaphylaxis, but we just don't understand it yet. What we can say for sure is this: IgA deficiency is a significant risk factor for anaphylactic transfusion reactions, with somewhat murky pathophysiologic mechanisms.
Wait a minute! Can we really say even that? We do have a little bitty problem with the definition of IgA deficiency. If you went and asked an allergist to give you a number for the frequency of selective IgA deficiency, you would hear something in the range of 1 in 700 people (the most common primary immunodeficiency of humans!). We know that anaphylactic reactions occur with much less frequency (1 in 20,000 to 1 in 50,000 units), so what is the deal? Blood Bankers believe that severe anaphylactic reactions occur in patients who aren't just IgA deficient, but are pretty well IgA-depleted. So, to us, quantity matters!
You should also know that IgA-deficient patients are not the only ones that get anaphylactic reactions. Similar reactions have been reported in patients who lack haptoglobin and certain complement components, notably portions of C4. These reactions presumably occur by a similar mechanism to that seen in IgA deficient patients (which is a really stupid thing to say, since we don't really understand that mechanism!).
Now, the treatment of an anaphylactic reaction.
Back to Transfusion Reaction Types