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The cells of the immune system have a wide range of functions: some are general cells which produce chemical signals to direct and control the response, while others are more active, seeking out and destroying antigens and the pathogens which manufactured them.
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Antigen-presenting cells (APCs) are those which present antigen on their surfaces. When antigen is presented in this way, it can activate other types of immune cells, causing them to start mounting an immune response to the antigen it recognized on the surface of the APC.
There are two general types of APCs: those which are specialized in their function, and do nothing but present antigen, and those which have dual or multiple functions.
The most specialized type of APC is the dendritic cell, and it is also thought to be the most important, and the one most capable of triggering other immune cells to mount a strong immune response. Other antigen-presenting cells include the macrophage, which is also a type of phagocyte.
While APCs are specialized immune cells, it’s also true that all cells in the body present antigen on their surfaces, and this is an important facet of the immune response. Host cells which are infected present non-self antigen on their surfaces, and this acts as a signal to immune cells that the cell is infected and must be destroyed.
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There are two categories of T-cells: helper T cells, and cytotoxic T-cells. Helper cells are so-called because they are the cells which coordinate an immune response, chiefly by secreting cytokines which provide chemical signals which tell other immune cells and organs what to do in the presence of a pathogen.
The role of the cytotoxic cell is much less refined, but equally important. These cells attack and kill host cells which are infected by pathogens
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B-lymphocytes, also simply called B-cells, are those which secrete antibody, chiefly in response to extracellular pathogens such as bacteria. Antibodies circulate through the bloodstream, but are completely unable to penetrate cells, and the task of actively attacking infected cells is left up to other immune cell types.
However, antigen produced by B-cells form complexes with bacterial cells, and this makes the foreign invaders better able to be targeted and destroyed by other types of immune cells.
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Phagocytes are another category of cell which kills infected cells, but they also directly kill pathogens – generally bacteria. Their method of killing such cells is simply to engulf the entire cell, essentially swallowing and digesting it.
Included in this category are macrophages, and this is why they are also antigen-presenting cells: when a macrophage engulfs a bacterial cell, it presents non-self antigen on its surface, thus acting as a trigger to other immune cells which recognize the antigen.
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These cells contain within them granules which are filled with chemicals with which they destroy pathogens. The neutrophil, for example, ingests cells and then releases the contents of its granules to kill them. Esosinophils and basophils do the opposite: rather than engulfing cells, they simply ‘spray’ their chemicals onto pathogens or infected cells.
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Natural Killer Cells
Natural kill cells (NK cells) are another type of lymphocyte, and like the cytotoxic cells, their main job is to directly kill cells. However, while cytotoxic cells generally kill infected host cells, NK cells are able to take the more direct route, and kill bacterial cells.