The plasma proteins of the complement system can be seen as the neighborhood watch whom sound the alarms to recruit back up. They are part of the body’s first defense mechanisms and are produced by the liver and found in the the blood and lymph. A majority of these soluble proteins are proteases. When in a dormant, quiescent state, they are referred to as zymogens. Upon infection by bacteria, they are activated and start a succession of enzymatic reactions where cleaving takes place (Janeway). What generally occurs is once a complement protein is activated, a piece or fragment of it covalently bonds to the pathogen essentially tagging it. The remaining piece of the protein recruits and attaches to a phagocytic white blood cell; it acts as a receptor to receive and bind to the fragment attached to the pathogen which acts as a ligand. The ligand and pathogen are then internalized via phagocytosis and enclosed by a phagosome that destroys the bacteria.
The activated protein just mentioned is called C3, one of the most important complement proteins(Janeway). The other alternatives to phagocytosis is the recruitment of inflammatory cells or the formation of the membrane-attack complex. The three ways to trigger the activation of the complement proteins are antigen-antibody complexes, lectin binding to the surfaces of pathogens, or the “physiochemical” changes in the environment caused by bacterial surfaces (Janeway). The complement system was discovered in the 19th century by Jules Bordet. He demonstrated that immune lysis requires the presence of a lytic factor that is easily altered by heat and able to kill bacteria, originally known as the “alexin” The other was a factor that is heat-stable known as the sensitiser or antibody (Nesargikar).
Besides the roles described above, the complements system has other plus factors that make it so essential to our immune system. One of them is how it increases blood flow and vascular permeability at the site of the infection. With this, blood plasma can enter the connective tissue and begin to heal. So it not only kills the bacteria but can initiate the healing of the injury.Systemic Lupus Erythematosus (SLE) is a chronic inflammatory autoimmune disease that attacks the skin, heart, Kong’s, kidneys, joints, and nervous system (Kuhn). SLE occurs from a deficiency in the components in the classical pathway of activation. Specifically, it is a deficiency in C1q, C1r/C1s or C4.
C1q is important because the pathway can be initiated by the binding of antigen-antibody complexes to C1q protein. When this C1 complex has been formed, conformational changes take place that activate C1r (Lintner). Activated C1r cleaves and activates C1s which then cleaves C4 into two fragments and C2 into two fragments. C4b and C2b bind and make C3, a big player in bacteria demolition. Though there is no cure for SLE, it has been suggested that the initial steps of the classical pathway have a key role in “achieving immune tolerance or preventing autoimmunity” (Lintner).