The complement system is made up of a series of 18 plasma proteins that are sequentially activated to mediate their biologic function. This function is to amplify specific and non-specific host defense, helping to mediate such functions as immune adherence, phagocytosis, chemotaxis and cytolysis. The classic activation pathway comprises the components C1, C4 and C2 and is dependent on the interaction of these complement proteins with antigen-antibody complexes to activate the initial cleavage of C3.
The alternative pathway activation occurs on mammalian cell surfaces in the absence of specific antigen-antibody complexes. This non-specific activation is a major physiological advantage since host protection can be generated prior to the induction of humoral immune response. Factors capable of activating the alternate pathway include zymosan, inulin, bacterial polysaccharides and endotoxins, IgG4, IgA and IgE. Thus the alternative pathway probably represents an earlier, more generalized defense system which is now an auxiliary pathway to the activation of extremely potent complement enzymes within the blood, which come into contact with the outer membrane of the invader. This chemical reaction essentially "shoots holes" in the membrane of the invader, causing it to burst or lyze. When not in use the various chemicals of the alternate complement pathway lay in a sort of suspended animation inside the blood. Their activation is a chain of events of sorts and is irreversible.
Stimulation of the alternate complement pathway also results in the production of several "intermediary" chemicals with special activity within the cardiovascular system such as serotonin, histamine and bradykinin. These chemicals cause an increase in the permeability of the capillary linings, allowing white blood cells to migrate from the tissues into the blood vessels. Chemotaxic hormones cause the white blood cells to be attracted and migrate towards the site of complement release. Serotonin can also act upon the brain and central nervous system while bradykinin is one of the most powerful inflammatory chemicals yet discovered and has been implicated in a wide variety of allergic reactions.
When the complement pathways are activated by existent infectious organisms, the system works quite well. But it is noteworthy that aggregated immunoglobulins to diverse elements -endotoxins, Candida, IgG4 mediated complexes (allergens and lectins) will summate and potentially augment C3b INA and C6 INA mediated ampliflication. Very often the highly destructive complement enzymes will turn upon the body itself, initiating autoimmune reactions which further elevate the immunoglobulin aggregate levels by increasing immune complex precipitation.
Thus the prognosis of any treatment rationale developed to combat inflammatory response to any single element in the bowel ecosystem will be in direct proportion to the degree of modulating the balance established between all correlating elements.