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Scheme of the complement system. The complement system, also known as complement cascade, is a part of the humoral, innate immune system and enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. [1]
The classical pathway is distinct from the other complement pathways in its unique activation triggers and cascade sequence. Activation of the complement pathway through the classical, lectin or alternative complement pathway is followed by a cascade of reactions eventually leading to the membrane attack complex.
The alternative pathway is a type of cascade reaction of the complement system and is a component of the innate immune system, a natural defense against infections. The alternative pathway is one of three complement pathways that opsonize and kill pathogens. The pathway is triggered when the C3b protein directly binds a microbe. It can also be ...
Structure of mannose in its α-D mannopyranose form. Mannan is a polymer of mannose.. The lectin pathway or MBL pathway is a type of cascade reaction in the complement system, similar in structure to the classical complement pathway, [1] in that, after activation, it proceeds through the action of C4 and C2 to produce activated complement proteins further down the cascade.
Two pathways of the complement cascade. [15] Components and enzymes of the classical and alternative pathways of the complement cascade, which provides a complementary means for human and other systems to defend against foreign pathogens (see text). Not shown are elements of the lectin pathway. [16]
Activation of the C1 complex initiates the classical complement pathway. This occurs when C1q binds to antigen-antibody complexes. The antibodies IgM or certain subclasses of IgG complexed with antigens are able to initiate the complement system: a single pentameric IgM can initiate the pathway, while several monomeric IgG molecules are needed. [3]
In contrast, any individual with increased complement levels or activity would have an elevated CH50 since increasing dilution would be necessary to reach the 50% lyse marking. Decreased CH50 values may be seen in cirrhosis or hepatitis [ 6 ] as a result of impaired complement production in the liver.
Complement component 3, often simply called C3, is a protein of the immune system that is found primarily in the blood. It plays a central role in the complement system of vertebrate animals and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3. [5] [6]