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The antigens and antibodies combine by a process called agglutination. It is the fundamental reaction in the body by which the body is protected from complex foreign molecules, such as pathogens and their chemical toxins. In the blood, the antigens are specifically and with high affinity bound by antibodies to form an antigen-antibody complex.
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 classical complement pathway can be initiated by the binding of antigen-antibody complexes to the C1q protein. The globular regions of C1q recognize and bind to the Fc region of antibody isotypes IgG or IgM. [2] These globular regions of C1q can also bind to bacterial and viral surface proteins, apoptotic cells, and acute phase proteins. [5]
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]
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]
These antigens can be visualized using a combination of antigen-specific antibody as well as a means of detection, called a tag, that is covalently linked to the antibody. [1] If the immunolabeling process is meant to reveal information about a cell or its substructures, the process is called immunocytochemistry . [ 2 ]
The complex is believed to be unstable until it binds properdin, a serum protein. The addition of properdin forms the complex C3bBbP, a stable compound which can bind an additional C3b to form alternative pathway C5-convertase. The C5-convertase of the alternative pathway consists of (C3b) 2 BbP (sometimes referred to as C3b 2 Bb).
It has recently been recommended as a fast and cost-effective epitope mapping approach, [35] using the complex protein system influenza hemagglutinin as an example. Cross-linking-coupled mass spectrometry. [36] Antibody and antigen are bound to a labeled cross-linker, and complex formation is confirmed by high-mass MALDI detection.