Natural variation in immune responsiveness, with special reference to immunodeficiency and promoter polymorphism in class II MHC genes

This review deals with natural selection operating on heterozygotes as a ke y factor controlling (a) the frequency of immunodeficiencies, and (b) promo ter polymorphism in MHC class II genes. The known difference in frequency d istribution of X-linked and autosomal deficiencies lend support to this pos sibility, and suggest chat the frequency of neonatal defect may rise as old -established equilibria between entry and exit of deleterious mutations cha nge. MHC class II gene promoters differ in their capacity to favor Th1 (or reciprocally Th2) responses, thus suggesting chat promoter polymorphism is sustained by the greater flexibility in response that this confers on heter ozygotes. Human Immunology 61, 177-181 (2000). (C) American Society for His tocompatibility and Immunogenetics, 2000. Published by Elsevier Science Inc .