Adaptive immune cells are more specialized, with each adaptive B or T cell bearing unique receptors, B-cell receptors (BCRs) and T-cell receptors (TCRs), that recognize specific signals rather than general patterns. Each receptor recognizes an antigen, which is simply any molecule that may bind to a BCR or TCR. Antigens are derived from a variety of sources including pathogens, host cells, and allergens. Antigens are typically processed by innate immune cells and presented to adaptive cells in the lymph nodes.
The genes for BCRs and TCRs are randomly rearranged at specific cell maturation stages, resulting in unique receptors that may potentially recognize anything. Random generation of receptors allows the immune system to respond to new or unforeseen problems. This concept is especially important because environments may frequently change, for instance when seasons change or a person relocates, and pathogens are constantly evolving to survive. Because BCRs and TCRs are so specific, adaptive cells may only recognize one strain of a particular pathogen, unlike innate cells, which recognize broad classes of pathogens. In fact, a group of adaptive cells that recognize the same strain will likely recognize different areas of that pathogen.
If a B or T cell has a receptor that recognizes an antigen from a pathogen and also receives cues from innate cells that something is wrong, the B or T cell will activate, divide, and disperse to address the problem. B cells make antibodies, which neutralize pathogens, rendering them harmless. T cells carry out multiple functions, including killing infected cells and activating or recruiting other immune cells. The adaptive response has a system of checks and balances to prevent unnecessary activation that could cause damage to the host. If a B or T cell is autoreactive, meaning its receptor recognizes antigens from the body's own cells, the cell will be deleted. Also, if a B or T cell does not receive signals from innate cells, it will not be optimally activated.
Immune memory is a feature of the adaptive immune response. After B or T cells are activated, they expand rapidly. As the problem resolves, cells stop dividing and are retained in the body as memory cells. The next time this same pathogen enters the body, a memory cell is already poised to react and can clear away the pathogen before it establishes itself.
Source: NIAID (NIH)1
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Adaptive Immunity: Protection from an infectious disease agent that is mediated by B- and T- Lymphocytes following exposure to specific antigen, and characterized by Immunologic Memory. It can result from either previous infection with that agent or vaccination (Immunity, Active), or transfer of antibody or lymphocytes from an immune donor (Immunization, Passive).2
Read about these related anatomy topics:
- T-cell receptors
- Lymph nodes
- Immune system
- T Cells
- Immune memory
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- Source: NIAID (NIH): niaid.nih.gov/ topics/ immunesystem/ Pages/ features.aspx
- Source: MeSH (U.S. National Library of Medicine)
- Source: NIAMS (NIH): niams.nih.gov/ Health_Info/ Sports_Injuries/ default.asp
- Source: MedLinePlus (NIH): medlineplus.gov/ infectiousdiseases.html
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