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).

desArg = without arginine

C3a desArg is a cleavage product of C3 complement component activation. Elevated levels of C3a have been reported in patients with acute lyme disease, acute pancreatitis, systemic lupus erythematosus, and adult respiratory distress syndrome.

C4 Complement, an essential component of the immune system's complement pathway, plays a pivotal role in both innate and adaptive immunity. Similar to C3, C4 is involved in the activation of the complement system, particularly in the classical and lectin pathways. Activation of C4 leads to its cleavage into C4a and C4b, with the latter playing a key role in opsonization (coating pathogens to promote phagocytosis) and in the formation of the C3/C5 convertase, crucial for further complement activation. Measuring the levels of C4 in the blood is an important diagnostic tool, especially in autoimmune and inflammatory conditions.

C4a Level by RIA (Radioimmunoassay) is an important test in immunology, offering insights into the body's immune response, particularly in the context of inflammation and autoimmune disorders. The C4a component is a part of the complement system, a group of proteins in the blood that play a crucial role in the body's defense against infections and in the process of inflammation. When the complement system is activated in response to a threat like an infection, C4a is produced as a byproduct. Measuring the levels of C4a, therefore, can help in understanding if the complement system is overactive, which is often the case in autoimmune diseases or during certain allergic reactions.