Circulating complexes in leprosy studied by the platelet aggregation test. The platelet aggregation test and its relation to the Rubino test and other sero-immunological parameters in 135 patients with leprosy.

Sera from 135 patients with leprosy were tested by the platelet aggregation test (PAT), by the Rubino test and by other sero-immunological assays. PAT positivity (titre≥10) was 53% in the lepromatous subgroups and 5% in the tuberculoid subgroups (P<0·005). The higher PAT titres and Rubino titres clustered significantly (P<0·0005) toward the lepromatous end of the disease spectrum. A statistically significant correlation was found between the PAT and the Rubino titres (0·05>P>0·025). Removal of the effect of the disease spectrum, however, resulted in a partial correlation between the PAT and the Rubino titres that was not significant (P>0·1), suggesting different basic mechanisms for the platelet aggregation (PA) and the Rubino activity of the lepromatous sera. The correlation between the PAT titres and twenty-nine other sero-immunological parameters was calculated, and a highly significant correlation was found between the PAT and the IgG level (P<0·005) and between the PAT and the antistaphylolysin-α titre (P<0·005). The PA activity in most lepromatous sera studied sedimented in the heavy (>19S) fractions and was inhibitable by IgM rheumatoid factor. It thus fulfilled the criteria for IgG complexes as defined in previous studies with known model Ag/Ab complexes and with sera from patients with immune complex states. The addition of an excess of soluble mycobacterial antigens affected the PA activity of some lepromatous sera, which suggests that the putative complexes were composed of mycobacterial antigens complexed with corresponding IgG antibody. It was concluded that the PAT is a sensitive detector of IgG complexes peculiar to the lepromatous leprosy. In leprosy the discriminatory power of the PAT seems to be superior to that of other immune complex tests recently applied for the analysis of leprosy series.