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The immune system is a hugely complex system which relies on tissues, cells, and molecules to function correctly. There are many different types of immune cell, each with specific functions, and there are immune organs where these cells interact directly with one another. The cells also communicate non-directly by means of chemical signals, various combinations of which are released by cells according to their situation and requirements.
One of the most important aspects of the entire system is its ability to distinguish between the body’s own cells, and the microscopic pathogens which invade them, referred to respectively as ‘self’ and ‘non-self.’ Whether self or non-self, anything that can trigger an immune response is called an antigen.
Antigens are small pieces of protein, usually between nine and eleven amino acids long, which are recognized by cells of the immune system. Under normal circumstances, only non-self antigens activate the immune system. The system has evolved such that when it works correctly, self antigens provoke a response called anergy, which essentially is a non-response.
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What Happens during an Infection?
When the body is invaded by a pathogen, such as a bacteria or virus, several things happen. The invading microorganism starts to multiply either within cells, or in the spaces between them, depending on the specific pathogen which is involved.
When the microorganism begins multiplying, it starts to produce proteins and other molecules which are released within cells or outside them. These non-self antigens are recognized by immune cells, thus beginning a complex cascade of events which results in an immune response that destroys the pathogen.
Of course, it’s nowhere near as simple as that. What actually happens during an immune response is far more complicated, and is dependent on several things, including the health of the immune system, the type of pathogen involved, and whether that particular pathogen has been ‘seen’ by the immune system before.
However, while other factors are involved, what happens when the immune system is activated depends largely on what type of pathogen has triggered the response. The immune response that occurs as the result of a bacterial infection is very different from the one which occurs as the result of a viral infection. The response is different again when allergies are involved (and the allergic response is an abnormal response rather than an attack against a pathogen).
Depending on the nature of the response, one or more of several types of immunological defense may be involved. Antibodies, for example, are generated in response to extracellular pathogens (those which grow outside of host cells), while cytotoxic cells are produced in response to intracellular pathogens (which grow inside host cells).