complement component c4

Complement component C4 is one of the components in the intricate complement system, which is encoded by two closely linked genes (C4A and C4B) located within the major histocompatibility complex (MHC) on chromosome 6p21.3. Because the C4A-C4B genes are two-locus allelic model, the genetics of human complement C4 is characterized with an abundant variation in the gene copy number and gene size within a population.

It is documented that the size of C4A or C4B gene can be 21 kb (long, L) or 14.6 kb (short, S), and the encoded proteins share 99% identity and differ by four isotype-specific amino acids in exon 26. Among different healthy individuals, 2 to 7 copies of C4 genes in a diploid genome are frequently present, which may increase the diversity and reduce the possibility of a total deficiency of C4A and C4B proteins in one subject.