The reason that why complement system is a critical component of antibody-mediated defense. Also, explain possible outcomes in a person if complement component C3, C5, and factor B of the alternative pathway and mannose-binding lectin pathway is absent in that particular individual. Concept introduction: The complement system comprises many soluble proteins which help in catalyzing the process of cell lysis and bacterial opsonization. The complement system is triggered by various types of interactions with antibodies and nonspecific activators such as mannose-binding lectin .

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Chapter 26, Problem 4AQ

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The reason that why complement system is a critical component of antibody-mediated defense. Also, explain possible outcomes in a person if complement component C3, C5, and factor B of the alternative pathway and mannose-binding lectin pathway is absent in that particular individual.

Concept introduction:

The complement system comprises many soluble proteins which help in catalyzing the process of cell lysis and bacterial opsonization. The complement system is triggered by various types of interactions with antibodies and nonspecific activators such as mannose-binding lectin.

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Mannose binding lectins (MBL) and ficolins are the two classes of proteins that can initiate the lectin pathway of complement activation. These proteins are selective for activating complement on the surfaces of microbial pathogens rather than host cells because: Their higher-order oligomeric structure can be assembled only after the monomers first bind to pathogen membranes. They only recruit MASP (MBL-associated serine proteases) proteins when bound to pathogen surfaces and not when bound to host cells. They only undergo the conformational change needed to activate MASP proteins when bound to a pathogen and not when bound to a host cell. They only bind to carbohydrate side chains and oligosaccharide modifications found on pathogen surfaces but not on host cell membranes. The activated MASP proteins are rapidly inactivated by hydrolysis when present on the surface of a host cell.

The formation of the C3 convertase is a key step in complement activation that occurs in all three complement pathways. This enzyme cleaves C3 in blood plasma, leading to a conformational change in the C3b fragment that exposes its reactive thioester group. The activated C3b is potentially harmful to the host, if it becomes covalently attached to a host cell, rather than to the surface of a pathogen. This deleterious outcome is largely avoided by: The inability of active C3b to diffuse away in the blood plasma. The inability of active C3b to covalently attach to the membranes of eukaryotic cells. The rapid hydrolysis of active C3b in solution, rendering it inactive. The tight binding of active C3b to the C3 convertase. The ability of active C3b to recruit phagocytic cells.

Neutralizing antibodies are effective at preventing infection or toxicity mediated by pathogens or their toxic products. In fact, nearly all vaccines currently in use function by eliciting neutralizing antibodies. One example is the tetanus vaccine, in which neutralizing antibodies are generated against an inactivated form of the tetanus toxin (the tetanus toxoid). The most important feature of a neutralizing antibody is having high affinity for the antigen. being efficient at activating the complement cascade. having a high degree of multivalency, such as being a pentamer or hexamer of immunoglobulin monomers. being present at a high concentration in the circulation. 0 0 0 0