The Journal of immunology : official journal of the American Association of Immunologists
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The role of Ig classes and subclasses in complement activation has been investigated both in vitro and in experimental animals, but not in humans. This study was conducted to determine the immunologic events of post-transfusion anaphylaxis in humans, and the effects of immune complexes of different IgG subclass compositions on complement activation. The ability of immune complexes containing mixed IgG1 and IgG4 or IgG4 Ab only to activate complement was investigated in two patients with von Willebrand's disease (a congenital bleeding disorder). ⋯ In the other patient, formation of IgG4-vWF led to a lesser degree of complement activation and was associated with moderately severe anaphylaxis. Since neither patient showed any biochemical alterations indicating the involvement of mast cells or the contact phase of coagulation at any time, it is probable that the pathogenetic mechanism of the clinical syndrome was a direct effect of complement anaphylatoxins on vascular permeability and smooth muscle contraction. In both patients, IgG-vWF bound C4 and C3 (IgG4-vWF to a lesser extent than mixed IgG1- and IgG4-vWF), and this probably prevented serum sickness as a complication.