Our immune system is a network of cells, tissues and organs that protect us from invading pathogens, keeping us healthy and able to fight off many recurring infections. When babies are babies, they receive immune cells from their mothers through the placenta and breast milk (if they are breastfed).

Complement consists of more than 20 different serum proteins, which contribute to the body's defense through many different pathways. C3 and C4 are two major components of the complement system, helping to assess the function of the complement system and its role in immune response, infections, and autoimmune diseases.