Although at least 21 different serum proteins have thus far been identified as part of the classical complement pathway, one can look at it as a pathway that is primarily initiated by either IgG or IgM binding (not IgE or IgA) to an antigen and involves 11 major serum protein components.
IgG and IgM are classes of antibody molecules, as mentioned previously, one of the major defenses against microbes is the immune defenses' (Figure1) production of antibody molecules against that microbe. The "tips" of the antibody (the Fab portion have shapes that are complementary to epitopes - portions of microbial proteins and glycoproteins found on the surface of the microbe. The Fc portion of IgG and IgM can activate the classical complement pathway by enabling the first enzyme in the pathway, C1, to assemble.
Figure 1: Structure of an antibody
The reactions are as follows:
a. Typically to activate the classical complement pathway, IgG or IgM is made in response to an antigen. The Fab portion of IgG (2 molecules) or IgM (1 molecule) reacts with epitopes of that antigen. A protein called C1q first binds to the Fc portion of antigen-bound IgG or IgM after which C1r and C1s attach to form C1, the first enzyme of the pathway
b. The activated C1 now enzymatically cleaves C4 into C4a and C4b. The C4b then binds to adjacent proteins and carbohydrates on the surface of the antigen and then binds C2. The activated C1 cleaves C2 into C2a and C2b forming C4b2a, the C3 convertase . Now the the classical complement pathway is activateD. C3 convertase can now cleave hundreds of molecules of C3 into C3a and C3b.
c. Some molecules of C3b bind to C4b2a, the C3 convertase, to form C4b2a3b, a C5 convertase that cleaves C5 into C5a and C5b .
d. C5b binds to the surface of the target cell and subsequently binds C6, C7, C8, and a number of monomers of C9 to form C5b6789n, the Membrane Attack Complex (MAC).
As mentioned above, components of the complement pathways carry out 6 beneficial innate defense functions. These include:
C5a is the most potent complement protein triggering inflammation. It causes mast cells to release vasodilators such as histamine so that blood vessels become more permeable; it increases the expression of adhesion molecules on leukocytes and the vascular endothelium so that leukocytes can squeeze out of the blood vessels and enter the tissue (diapedesis); it causes neutrophils to release toxic oxygen radicals for extracellular killing; and it induces fever. To a lesser extent C3a and C4a also promote inflammation. As we will see later in this unit, inflammation is a process in which blood vessels dilate and become more permeable, thus enabling body defense cells and defense chemicals to leave the blood and enter the tissues.
B. Chemotactically attracting phagocytes to the infection site
C5a also functions as a chemoattractant for phagocytes. Phagocytes will move towards increasing concentrations of C5a and subsequently attach, via their CR1 receptors to the C3b molecules attached to the antigen. This will be discussed in greater detail later in this unit under phagocytosis.
C3b and to a lesser extent, C4b can function as opsonins, that is, they can attach antigens to phagocytes. One portion of the C3b binds to proteins and polysaccharides on microbial surfaces; another portion attaches to CR1 receptors on phagocytes, B-lymphocytes, and dendritic cells for enhanced phagocytosis. . Actually, C3b molecule can bind to pretty much any protein or polysaccharidE. Human cells, however, produce Factor H that binds to C3b and allows Factor I to inactivate the C3b. On the other hand, substances such as LPS on bacterial cells facilitate the binding of Factor B to C3b and this protects the C3b from inactivation by Factor I. In this way, C3b does not interact with our own cells but is able to interact with microbial cells. C3a and C5a increase the expression of C3b receptors on phagocytes and increase their metabolic activity.
C5b6789n, functions as a Membrane Attack Complex (MAC) . This helps to destroy gram-negative bacteria as well as human cells displaying foreign antigens (virus-infected cells, tumor cells, etc.) by causing their lysis; and . It can also damage the envelope of enveloped viruses.
Some C3b is converted to C3d. C3d binds to CR2 receptors on B-lymphocytes. This serves as a second signal for the activation of B-lymphocytes whose B-cell receptors have just interacted with their corresponding antigen.
F. Removing harmful immune complexes from the body
C3b and to a lesser extent, C4b help to remove harmful immune complexes from the body. The C3b and C4b attach the immune complexes to CR1 receptors on erythrocytes. The erythrocytes then deliver the complexes to fixed macrophages within the spleen and liver for destruction. Immune complexes can lead to a harmful Type III hypersensitivity.
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