It was shown that human IgG molecules have at least one antigenic determinant of the rabbit a1 specificity. This a1 determinant in man was present in all 270 individuals tested, was found on at least 60% of the IgG molecules, and was localised to the Fab fragment. Population studies with anti a1 as well as with anti a2 and anti a3 purified rabbit antibodies reacting with human IgG should resolve the genetic basis for these determinants in man, and indicate whether they are isotypic or allotypic antigenic determinants, that is present in all individuals or segregated among the population as in rabbits. Studies with myeloma proteins of different classes and subgroups should be useful for the precise localisation and characterisation of these determinants in human immunoglobulins.
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