The Third line of Defence
First:Antigens are seen as chemical tags that allow the body to determain if a molecule is part of the body or a foreign intruder. All cells in the body have antigens, and pathogens also carry antigens on their surface. The specific immune response of the third line of defense starts when a macrophage ingests the foreign invader and presents its antigens to a helper T cell. The helper T cell releases chemical signals which activate the rest of a specific set from the third line of defense (B-cells, Killer T cells) and cause them to multiply. Stem cells in the bone marrow are different from other body cells due to a difference occurring during mitosis. The daughter T-cells created from mitosis differentiate from one another by having slightly different antigen receptors, allowing the body to combat a large variation of diseases if a certain disease's antigen becomes present in the body.
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Second:The helper T cells go to activate killer T cells with proceed to destroy cells that have already been infected. It also activates B cells. Within you there are already millions of differentiated B cells ready and waiting to be activated to start fighting an invader. These B cells start producing antibodies which make the pathogens much easier to fight.
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Third:Not all of the all of the cells go to fight the infection, some become memory B and T cells. These memory cells stay inside of you and are ready to immediately start fighting this disease if you ever run into it again. The memory cells allow your body to skip the process of multiply from the parent T-cells that are activated by the antigen and allow for an immediate immune response.
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