C3 is the major complement component.  Associated with bacterial infections esp gram neg eg Meningococcus, Enterobacter, Haemophilus, E coli.  Gram pos infections also seen.  Some increase in autoimmune eg SLE but this is more well recognized with C1q, C2 and C4 def.

Secondary C3 def seen with nephritis (complement activation products in kidney), but also with hereditary factor I and H deficiency (cofactors in C3 product cleavage).