P0C0L4 

Complement C4 protein plays a crucial role in the complement system, which is essential for immune defense and inflammation regulation. C4 is synthesized as a precursor that is cleaved into three chains: α, β, and γ, which are non-identical cleavage products. The α chain can further be cleaved into two isotypes, C4a and C4b, each with distinct reactivity; C4a interacts with amino groups, while C4b reacts with hydroxyl groups. This functional diversity is vital for the complement system′s ability to target pathogens and modulate immune responses. Notably, the human C4 gene is polymorphic at two loci, C4A and C4B, located on chromosome 6p21.3, with C4A expressing the Rodgers blood group antigen and C4B expressing the Chido blood group antigen.