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Antibodies bind to pathogens to opsonize them, neutralize pathogen toxins, and activate the complement complex system. IgE also alerts circulating mast cells and eosinophils of known antigens, which causes a rapid inflamamatory response.
The adaptive immune system mounts a stronger, and antigen specific immune response after the innate immune response fails to prevent a pathogen from causing an infection. There are two subdivisions of the adaptive immune system: cell-mediated immunity, and humoral immunity.
Cell Mediated Immunity
Cell mediated immunity is controlled by type 1 helper T-cells (Th1) and cytotoxic T-cells. These cells are activated by antigen presenting cells, which causes the cells to rapidly mature into forms that are specific to that antigen. T-cells will then circulate through the body to destroy pathogens in several ways. Helper-T cells facilitate the immune response by guiding cytotoxic T-cells to pathogens or pathogen-infected cells, while the cytotoxic T-cells will kill the pathogens or pathogen-infected cells.
Some of the ways in which cytotoxic T-cells kill pathogens include the release of granules that contain the cytotoxins perforin and granzyme, which lyse small pores in the membrane of a pathogen. Then T-cell produced proteases can enter the pathogen and induce an apotosis response within the cell. Helper-T cells will secrete cytokines, such as interferon-gamma, which can activate cytotoxic T-cells and macrophages.
Humoral immunity refers to the component of the adaptive immune response that is caused by B cells, antibodies, and type 2 helper T-cells (Th2), as well as circulating mast cells and eosinophils to a smaller extent. Its name comes from the idea that blood is one of the humors of the body, because antibodies will provide passive or active immunity through circulation in the blood stream.
Type 2 helper T-cells are included in the humoral immune system because the present antigens to immature B-cells, which undergo proliferation to become specific to the presented antigen. The B-cells then rapidly produce a large number of antibodies which circulate through the plasma of the body.
Antibodies to provide a number of functions in humoral immunity. There are 6 different classes of antibodies that provide similar but different functions and interact with different cells in the immune system. The overall functions of antibodies are to bind to pathogens to opsonize them, which makes it easier for phagocytic cells to bind and destroy the pathogen. They can also neutralize the toxins produced by certain pathogens. Their other main function is complement pathway activation, in which circulating proteins are combined in a complex cascade that forms a membrane attack complex on a pathogen cell membrane, which lyses the cell.
Mast cells and eosinophils are considered to be part of the humoral immune system because they can be sensitized towards certain antigens through circulating immunoglobin E (IgE), which is a specific type of antibody produced by B cells. IgE will bind to the mast cells and eosinophils when an antigen is detected, using a type of Fc receptor on the mast cell or eosinophil that has a high binding affinity with IgE. This binding will cause degranulation and relase of inflammatory mediators that start an immune response against the antigen. This process is the reason why memory B cells can cause hypersensitivity (allergy) formation, as circulating IgE from those memory cells will activate a rapid inflammatory and immune response.
Source: Boundless. “Types of Adaptive Immunity.” Boundless Anatomy and Physiology. Boundless, 24 Jun. 2016. Retrieved 27 Aug. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/immune-system-21/adaptive-immunity-198/types-of-adaptive-immunity-976-10276/