T cells also participate in the adaptive immune response. Both B cells and T cells are lymphocytes, differentiating them from other white blood cells in the body. T cells don’t directly recognize a pathogen. Instead they recognize specific antigens that are attached to MHCs (major histocompatibility complexes) that are basically self-identifying complexes. These MHCs are necessary for T cells to recognize the antigen. When a cell is infected, antigens from the pathogen will be found on the outside of the cell, linked to MHCs that identify the cell as belonging to the self but being infected as well. These antigen-presenting cells will trigger a T cell and a B cell response that is specific to the pathogen. While the B cell will be triggered to make antibodies, the activated T cell will directly kill anything that contains the antigen they’ve been activated to kill. This is called cell-mediated immunity. T cells only kill self-cells that have been identified as being infected or possibly cancerous. There are cytotoxic T cells that do the killing of infected cells and T helper cells that aid in the process but do not do any infected cell killing. T helper cells will secrete cytokines when in contact with an infected cell. This stimulates NK cells and macrophages to kill the pathogen. So, how do vaccines work? They are given to humans and animals in order to prevent an infection. These represent killed bacterial cell wall parts, killed viruses, or live but harmless viruses that can generate an immune response against the infection they represent. These vaccines basically contain antigens that allow the adaptive immune system to kick in and to develop a memory for having an infection that the individual never actually had in the first place. This memory prevents the infectious organism from taking hold in the body by tricking the immune system into thinking the person has already had the infection. These will confer a long-term response against infection.
ENDOCRINE SYSTEMS Some aspect of an endocrine system exists in animals of all types. It represents several glands that secrete hormones. Hormones can be simple molecules, modified amino acids, and peptides that act over long distances to affect the body in some way. Hormones are secreted by a variety of endocrine glands that have no ducts but secrete
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