What is the mechanism of anaphylaxis? What is the difference between anaphylactic and anaphylactoid reactions?
Anaphylaxis is an IgE-mediated allergic reaction. Antigens crosslinking two molecules of IgE bound on a mast cell or a basophil trigger degranulation of the cell with release of fac-tors such as histamine, platelet-activating factor (PAF), eosinophil and neutrophil chemotactic factors, vasodilating prostaglandins and leukotrienes, adenosine, and serotonin. These factors in turn activate the complement and coagula-tion cascades. They also have direct effects, the most relevant being systemic arteriolar vasodilation, with an acute increase in vascular permeability and bronchoconstriction.
Anaphylactoid reactions are clinically indistinguishable from anaphylaxis. However, the mechanism of action differs in that IgE is not involved. Anaphylactoid reactions are a result of direct degranulation of mast cells and basophils with release of the same mediators as in anaphylactic reactions.