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The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM. [ 1 ] [ 2 ] Following activation, a series of proteins are recruited to generate C3 convertase (C4b2b, historically referred C4b2a), which cleaves the C3 protein.
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]
Three biochemical pathways activate the complement system: the classical complement pathway, the alternate complement pathway, and the mannose-binding lectin pathway. [9] These processes differ only in the process of activating C3 convertase, [10] which is the initial step of complement activation, and the subsequent process are eventually the ...
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 immunology, antibodies (immunoglobulins (Ig)) are classified into several types called isotypes or classes. The variable (V) regions near the tip of the antibody can differ from molecule to molecule in countless ways, allowing it to specifically target an antigen (or more exactly, an epitope). In contrast, the constant (C) regions only occur ...
An immune complex, sometimes called an antigen-antibody complex or antigen-bound antibody, is a molecule formed from the binding of multiple antigens to antibodies. [1] The bound antigen and antibody act as a unitary object, effectively an antigen of its own with a specific epitope .
The C1q domain is a conserved protein domain. C1q is a subunit of the C1 enzyme complex that activates the serum complement system.C1q comprises 6 A, 6 B and 6 C chains.These share the same topology, each possessing a small, globular N-terminal domain, a collagen-like Gly/Pro-rich central region, and a conserved C-terminal region, the C1q domain. [2]
The classical and alternative complement pathways. Alternative pathway. (Some labels are in Polish.) 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.