The effect of heat inactivation of serum on the detection of immune complexes

Heat inactivation has previously been shown to increase the reactivity of many pathological sera when tested for complement-fixing immune complexes. The mechanism of this effect has been presumed to be inactivation by heat of endogenous complement already bound to immune complexes. Our data demonstrate that immune complexes or immunoglobulin aggregates actually become less detectable by complement fixation when they are added to serum and then heated at 53°C or 56°C, while heating in saline does not alter their reactivity. Prior inactivation of factor B of the alternative complement pathway by heating at 50°C for 30 minutes does not prevent this effect of serum. Rather, the diminished detection of immune complexes or immunoglobulin aggregates appears to be due to an interaction of these substances with a heat-labile serum euglobulin which has characteristics similar to those of C1q. Heat is not essential for the effect of this euglobulin since a reduction of (125)I-C1q binding occurs when immunoglobulin aggregates are added to unheated serum. Heat inactivation, however, further reduces detection of immunoglobulin aggregates in serum using the (125)I-C1q binding test. These data indicate that 1. assays using complement fixation, although generally sensitive, do not optimally detect immune complexes in serum, and 2. heating at 53°C or 56°C, rather than making these complexes more detectable by inactivating endogenous bound complement, further diminishes their detectability. It is suggested that heating might increase reactivity in these assays by producing immunoglobulin aggregates in serum.