[Monoclonal myelomatous IgA with antilipoprotein antibody activity of Ra specificity (author's transl)]

Investigations performed in a patient with myeloma, hyperlipidemia, and xanthomatosis demonstrated the antilipoprotein activity of the monoclonal IgA directed against an antigenic site--called Ra.--different from those previously described. A complex IgA beta-lipoprotein has been firstly characterized. After isolation and purification of the IgA, it has been shown that the association between IgA and lipoprotein was immunologically mediated. The antibody is an IgA lambda bound via its Fab portion and in fixed combining ratio to an antigenic determinant shared only by LDL and VLDL of humans and some other mammalians to the exclusion of any other serum proteins. The results obtained with passive hemagglutination and inhibition of hemagglutination suggest that the antigenic site Ra. is not located on apoprotein B (major proteic moiety of LDL and VLDL), since the antigenic determinants of apolipoproteins are different in humans and in animals and since IgA Ra. fails to react with apolipoprotein B obtained by delipidation of LDL. On the other hand, the lack of reaction between IgA Ra. and HDL suggests that the antigenic determinant is not only present on the lipid hapten such as in the case of PG and AS determinants which are located on VLDL, LDL, and HDL from humans and animals. So the antigenic determinant revealed by IgA Ra. seems to be different from those previously described.