Complement is a complex innate immune surveillance system, playing a key role in defense against pathogens and in host homeostasis. The complement system is initiated by conformational changes in recognition molecular complexes upon sensing danger signals. The subsequent cascade of enzymatic reactions is tightly regulated to assure that complement is activated only at specific locations requiring defense against pathogens, thus avoiding host tissue damage.
The classical pathway is initiated by IgM or IgG antigen/antibody complexes binding to C1q (first protein of the cascade) leading to activation of C1r, which in turn cleaves C1s. This in turn activates the serine proteases that lead to cleaving of C4 and C2, leading to formation of C4b2a (C3 convertase), which in turn cleaves C3 into C3a and C3b. While C3a acts as a recruiter of inflammatory cells (anaphylatoxin), C3b binds to the C4b2a complex to form C5 convertase (C4b2a3b). The C5 convertase initiates the formation of the Membrane Attack Complex (MAC), that inserts into membrane creating functional pores in bacterial membranes leading to its lysis. The classical pathway can also be activated by other danger signals like C-reactive protein, viral proteins, polyanions, apoptotic cells and amyloid, thus providing evidence that classical pathway could be activated independent of antibodies.
Complement activation alternative pathway was discovered by Pillemer and colleagues in 1954 but was recognized universally some years later. The alternative pathway of the complement system is an innate component of the immune system's natural defense against infections. The alternative pathway is one of three complement pathways that opsonize and kill pathogens.This pathway is activated by viruses, fungi, bacteria, parasites, cobra venom, immunoglobulin A, and polysaccharides and forms an important part of the defense mechanism independent of the immune response. Here, C3b binds to factor B that is cleaved by factor D to Bb. C3bBb complex then acts as the C3 convertase and generates more C3b through an amplification loop. Binding of factor H to C3b increases its inactivation by factor I. Properdin stabilizes it, preventing its inactivation by factors H and factor I. The alternate pathway does not result in a truly nonspecific activation of complement because it requires specific types of compounds for activation. It simply does not require specific antigen-antibody interactions for initiation.
The lectin pathway is very similar to the classical pathway. It is initiated by the binding of mannose-binding lectin (MBL) to bacterial surfaces with mannose-containing polysaccharides (mannans). Binding of MBL to a pathogen results in the association of two serine proteases, MASP-1 and MASP-2 (MBL-associated serine proteases). MASP-1 and MASP-2 are similar to C1r and C1s, respectively and MBL is similar to C1q. Formation of the MBL/MASP-1/MASP-2 tri-molecular complex results in the activation of the MASPs and subsequent cleavage of C4 into C4a and C4b. The C4b fragment binds to the membrane and the C4a fragment is released into the microenvironment. Activated MASPs also cleave C2 into C2a and C2b. C2a binds to the membrane in association with C4b and C2b is released into the microenvironment. The resulting C4bC2a complex is a C3 convertase, which cleaves C3 into C3a and C3b. C3b binds to the membrane in association with C4b and C2a and C3a is released into the microenvironment. The resulting C4bC2aC3b is a C5 convertase. The generation of C5 convertase is the end of the lectin pathway. The biological activities and the regulatory proteins of the lectin pathway are the same as those of the classical pathway.
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