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The various arms of the immune system can be categorized in several different ways. One of these is in terms of antibody-mediated and cell-mediated immunity. Antibody-mediated immunity (also known as the type 2 immune response) involves the activation of B cells, and this arm of the immune system is commonly activated to deal with extracellular pathogens.
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B Cell Development
B cells develop in the bone marrow, and travel from there to the spleen. Once there, they undergo a maturation process during which the genes responsible for generating antibody recombine several times. This process renders the cells highly specific for a single antigenic sequence.
During maturation, each cell undergoes selection mechanisms which ensure it is not only specific for one antigen, but also that it does not recognize self antigen. During this process, any B cells which recognize self antigen either die or their activity is permanently suppressed.
When the B cell has finished the recombination process, it is fully mature. Once it has matured, the cell is at a stage where it will activate only when it recognizes a particular amino acid sequence during the course of a pathogenic infection. Mature B cells circulate throughout the body, via the bloodstream and lymphatic system, until they come into contact with the antigen they specifically recognize.
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Antibody Production and Memory Cells
When an infection begins, some of the B cells circulating throughout the body will recognize and present antigens which are produced by the invading pathogen. B cells which present antigens on their surfaces will then receive an activating signal from a helper T cell, along with cytokines secreted by type 2 helper cells. When this happens, those B cells become activated, and will divide continuously for four or five days. After this point, one of several things may happen.
Depending on the specific type of pathogen involved, the B cells may receive chemical signals from other immune cells and molecules, which cause the cells to respond in certain ways. Depending on the chemical instructions received, the B cell might become an antibody-producing plasma cell, or become a memory cell. It may also undergo a process called isotype switching, in which it becomes programmed to produce a particular class of antibody.
Plasma B cells are more or less antibody factories, doing little else but producing and secreting huge amounts of antibody. There are five different types of antibody: the kind produced by B cells typically depends on the specific type of pathogen which has invaded the body (the next part of this series will focus on these different types of antibodies, and their specific functions and effects).
Memory cells, on the other hand, do not produce antibody during an initial infection. Instead, they remain in the body for years—possibly an entire lifetime—dividing and persisting, ready to be activated if the same type of pathogen causes a subsequent infection.
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Antibodies also work in conjunction with another set of proteins which is known as the complement system. The complement system is a set of around 25 proteins which work with antibodies to destroy bacteria.
Complement proteins circulate within the bloodstream in an inactive form, and are activated when the first protein in the complement cascade is itself activated by an antibody-antigen complex.
When this happens, the cascade is triggered. Each of the proteins within the complement system is part of a cascade which eventually results in the formation of a complex protein molecule shaped like a cylinder, which inserts itself in the wall of a bacterial cell and causes it to burst.
An Overview of Antibody-Mediated Immunity in Humans
This five-part series looks at different types of immune responses, including innate and adaptive, and cell-mediated and antibody-mediated responses.