C. difficile is an opportunistic anaerobic bacterium which causes symptoms ranging from mild diarrhea to pseudomembranous colitis when the normal flora has been altered (as in antibiotic use).

C. difficile produces two toxins:

- Toxin A is a tissuedamaging enterotoxin,

- while toxin B is referred to as a cytotoxin.

C1 inhibitor is a multispecific, protease inhibitor that is present in normal human plasma and serum, and which regulates enzymes of the complement, coagulation, fibrinolytic, and kinin-forming systems. The enzymes (proteases) regulated by this protein include the C1r and C1s subunits of the activated first component of complement, activated XIIa, kallikrein (Fletcher factor), and plasmin.

Measurement of the C1 esterase inhibitor (the first component of the complement) is used to diagnose hereditary angioedema and to monitor levels of the inhibitor during treatment.

The complement system is a group of nearly 60 proteins in blood plasma or on the surface of some cells. The complement proteins work with your immune system to protect the body from infections. They also help remove dead cells and foreign material. There are nine major complement proteins. They are labeled C1 through C9.

In some rare cases people may inherit deficiency of some complement proteins. These people are prone to certain infections or autoimmune disorders.

C3 Complement, often simply referred to as C3, is a crucial protein component of the immune system's complement system. The complement system is a part of the innate immune response and plays a significant role in inflammation and defending against infections. C3, in particular, is central to the activation of the complement system. It gets activated through one of the three complement pathways (classical, lectin, or alternative) and subsequently participates in opsonization (marking pathogens for destruction), chemotaxis (attracting immune cells to the site of infection), and cell lysis (breaking down of cells).