Laboratory detection of complement activation and complement deficiencies.

The complement system is the major humoral amplification and effector mechanism of the immune system. Complement is activated in a variety of conditions, especially in immune-complex diseases. Activation can result in consumption of components involved in either the classical or the alternative complement pathway or both pathways. Such activation can be detected by laboratory analysis, quantitating specific complement component levels and measuring hemolytic function. An integrated approach to the laboratory evaluation of complement can ascertain which complement pathway is being activated, either in vivo or in vitro. Complement deficiency states are not rare, and a large enough number of such patients have now been detected for the patterns of disease susceptibility to be recognized. Since inherited deficiency of a complement protein may have dire consequences, it is important for laboratory technologists to recognize the patterns of reactivity associated with these deficiency states. The physician can then be alerted so that appropriate and effective preventive or therapeutic measures can be taken. In addition, the study of patients lacking an individual complement component has provided substantial understanding of the role of the complement system in host defense and inflammation.