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The immune system has three key abilities. First is the ability to detect entities that might do us harm. Innate to this system is the ability to recognize antigens at the molecular level, and antibodies and T cell receptors are the key proteins. Each antibody or T cell receptor recognizes a unique shape and this variability is achieved through the random mixing of genes during the differentiation of T and B cells. Second, the immune system is tolerant. Since antibody and T cell receptor differentiation is a random process, immature B cells and T cells are created that react with our own macromolecules. These potentially lethal cells are identified and nullified by a number of mechanisms. This last part is the key step that allows the immune system to only attack antigens that are non-self. Third, the immune system is adaptive. After an initial challenge by an antigen, the immune system has a memory in the form of the aptly named memory cells. A second challenge by a similar antigen elicits a more rapid and stronger immune response and, in the case of a pathogen, prevents it from causing disease. One great advance in treating disease has been the activation of the immune system against a pathogen by presenting its antigens, but in a form that does not cause disease. We are, of course, referring to vaccination and its use has been one of the major methods of preventing infectious diseases, as we will describe in Chapter 17.
Our bodies are constantly bombarded with trillions of viruses and billions of microorganisms. To withstand this onslaught, multicellular organisms evolved various abilities to defend themselves against these invaders. While these pathogens are resourceful and persistent, the multi-faceted nature of the immune system and its adaptability successfully fends off these attacks. The immune system is critical to our survival and the constantly increasing knowledge that we have of its function will likely lead to many revolutionary treatments in the future.
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