When Mallory was asked why he wished to climb Mount Everest he answered “because it’s there”. Similarly complement genetics should be studied because there is so much genetic variation there to study. Table 1 shows how common genetic polymorphisms are among human complement components. The term “polymorphism” was coined by E.B. Ford (1) to denote an allelic system where the second most common allele occurs at a frequency too high to be maintained by mutation alone (generally taken as a frequency above 1%). It is generally held that polymorphisms are maintained in the population by selection, the heterozygote having some selective advantage to compensate for any disadvantage of the less favourable allele. The classic example is the case of haemoglobin S where the heterozygote enjoys some protection against death from malaria in the early months of life, which compensates for the considerable disadvantage of homozygous sickle cell disease. It is with this example in mind that the existence of the complement polymorphisms (and particularly of high frequency polymorphisms where the frequency of the second allele exceeds 10% -which occurs for C3, C6, Factor B and C4−) has led to a long and hard search for functional differences between the different alleles and for association of individual alleles with diseases in man.
KeywordsComplement Component Complement Deficiency Complement Locus Serine Esterase Nephritic Factor
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