Specific antibodies are formed that can recognize and bind the antibody to result in adaptive immunity against an antigen.
Adaptive immunity is triggered when a pathogen evades the innate immune system for long enough to generate a threshold level of an antigen. An antigen is any molecule that induces an immune response, such as a toxin or molecular component of a pathogen cell membrane, and is unique to each species of pathogen. A typical adaptive immune response includes several steps:
The antigen for the pathogen is taken up by an antigen-presenting cell (APC), such as a dendritic cell or macrophage, through phagocytosis. The APC travels to a part of the body that contains immature T and B cells, such as a lymph node.
The antigen is processed by the APC and bound to MHC class II receptors and MHC class I receptors on the cell membrane of the APC. The antigen is presented to immature helper T cells and cytotoxic T cells through binding the MHC II (helper T) or MHC I (cytotoxic T) to T-cell receptors. These T lymphocytes mature and proliferate. Helper T cells activate B cells, which proliferate and produce antibodies specific to the antigen. In contrast, cytotoxic T cells destroy pathogens that bear the antigen that was presented to them by the APCs. Memory B and T cells are formed after the infection ends.
Practice Questions
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Key Points
• The cells of the adaptive immune system are a type of leukocyte called a lymphocyte. B cells and T cells are the major types of lymphocytes involved in adaptive immunity.
• B and T cells can create memory cells to defend against future attacks by the same pathogen by mounting a stronger and faster adaptive immune response against that pathogen before it can even cause symptoms of infection.
• Antigen-presenting cells present captured antigens to immature lymphocytes, which then mature to be specific to that antigen and work to destroy pathogens that express that antigen.
• Specific antibodies are then produced to fight against the antigens.
Key Terms
Antigen: a substance that induces an immune response, usually a molecule found on a pathogen such as a toxin or a molecule expressed by the pathogen or pathogen-infected cells.
Antigen-presenting cell: a cell that presents captured antigens to immature T-cells. Dendritic cells and macrophages are the best examples, but several other cells can present antigens as well.
Memory T cells: a type of T cell that rapidly differentiates into helper and cytotoxic T cells if its associated antigen is detected.
MHC I: molecules are expressed on all nucleated cells and are essential for the presentation of normal “self” antigens. Cells that become infected by intracellular pathogens can present foreign antigens on MHC I as well, marking the infected cell for destruction.
MHC II: molecules are expressed only on the surface of antigen-presenting cells (macrophages, dendritic cells, and B cells). Antigen presentation with MHC II is essential for the activation of T cells
Antibody: protein produced in the blood to fight against an antigen
Adaptive immunity: a specific type of immunity developed over time
Pathogen: any organism or substance, especially a microorganism, capable of causing diseases, such as bacteria, viruses, protozoa, or fungi
Cytotoxic T cell: a T lymphocyte that kills cells that are damaged
Phagocytosis: the process by which certain living cells called phagocytes ingest or engulf other cells or particles
B cell: a lymphocyte, developed in the bursa of birds and the bone marrow of other animals, that produces antibodies and is responsible for the immune system