Activation of the alternative pathway of complement by skin immune deposits.

Skin immune deposits at the basement membrane zone have been demonstrated by functional assays to activate complement. This important biologic function has not yet been explored for immune deposits present in other locations mainly because many cytoplasmic structures in the skin have the capacity to activate the complement cascade by the classical pathway. In this study the capacity of immune deposits to activate directly the alternative pathway was examined using a functional guinea pig C3 binding test. This test was devised so as to avoid complement activation by normal cutaneous structures, thus it did not examine the capacity of immune reactants to activate the classical pathway. The main findings were that alternative pathway activation could be demonstrated only when human C3 deposits were seen by direct immunofluorescence, but not all C3 deposits were found to activate the alternative pathway; such activation was restricted to vascular deposits; the phlogistic potential of the immune deposits correlated with serologic evidence of ongoing immune reactions, i.e., hypocomplementemia and circulating immune complexes. It is suggested that this test provides data on one aspect of the phlogistic potential of skin immune deposits not detectable by direct immunofluorescence.