The role of C3 in the immune response.

The complement system, particularly the third component, plays an important modulatory role in the inductive phase of the immune response. As discussed here by Anna Erdei and colleagues, the picture that is emerging is that immobilized C3 split products facilitate the cooperation between immunocompetent cells and are co-stimulatory molecules in T- and B-cell activation, probably as a result of their ability to promote cell-cell adhesion. In contrast, soluble C3 products inhibit lymphocyte proliferation.