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An immune system's coordinated response to prevent infection.
Updated May 10, 2022.
The function of the immune system is to protect animals from foreign agents and infectious organisms. It responds to pathogens in a specific way and can display a long-term memory of infectious agents' exposure. The immune system consists of two functional components:
The innate immune system components provide the first line of defense against infection. Physical barriers to infection include skin, which prevents pathogen penetration, and bodily fluids, like mucus, which collect and clear pathogens.
Many cellular and biochemical components, including complement proteins, innate leukocytes, and phagocytic cells, identify and eliminate pathogens from the body.
The innate immune system's function and efficiency do not change with repeated exposure to foreign pathogens.
The adaptive immune system is activated when the innate system fails to clear pathogens from the body. It consists of various cells and molecules, with lymphocytes and antibodies being the key elements.
Lymphocytes arise continuously from progenitor cells in the bone marrow. Lymphocytes synthesize cell surface receptors or secrete proteins that specifically bind to foreign molecules. These secreted proteins are known as antibodies. Any molecule that can bind to an antibody is called an antigen. The term antibody is used interchangeably with immunoglobulin.
Pathogens bound to antibodies are marked for clearance or destruction.
Most functions of the adaptive immune system can be described by grouping lymphocytes into three basic types:
The adaptive immune response can be either humoral or cell-mediated. B lymphocytes mediate the humoral response by releasing antibodies specific to the infectious agent. The cell-mediated response involves binding TC cells to foreign or infected cells, followed by the lysis of these cells.
Th cells are involved in both responses through the release of cytokine proteins. All three types of lymphocytes carry cell surface receptors that can bind antigens. All antigen receptors are glycoproteins, and only one kind of receptor is synthesized within any one cell. The specificity of the immune system is impacted by the fact that one cell recognizes only one antigen.
The antibody-antigen interaction forms the basis of all immunoassays but is also the basis for the immune response.
The region of the antibody that reacts with the antigen is called the paratope. The region of an antigen that interacts with an antibody is defined as an epitope. Affinity measures the strength of the epitope's binding to an antibody and is often represented by the dissociation constant KD. Avidity measures the overall stability of the complex between antibodies and antigens.
An antibody response is the culmination of a series of interactions between macrophages, T lymphocytes, and B lymphocytes. Infectious agent antigens are engulfed and partially degraded by antigen-presenting cells (APCs), such as macrophages, Langerhans cells, dendritic cells, lymph nodes, and monocytes.
The antigen's fragments will appear on the APC's surface attached to a cell surface glycoprotein known as MHC II (major histocompatibility complex). There are two types of MHC molecules: MHC class I, expressed on the surfaces of most cells, and class II, expressed exclusively on APCs' surfaces. The antigen-MHC II complex allows Th cells to bind to the APC, leading to a proliferation of Th cells and cytokine release. T cells then bind to the MHC complex on B cells, leading to B cells' proliferation and differentiation. B cells change into plasma cells, secreting large quantities of finely tuned antibodies specific to the foreign agent. Some B cells are transformed into memory cells, allowing for a faster antibody-mediated immune response upon future infection.