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Old 06-11-2012
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Search Definitions of immunological terms needed

Can anyone define what is meant by clonal selection, clonal expansion, clonal deletion and anergy?

Thanks in advance...
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Old 06-11-2012
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Anergy - Functional inactivation of lymphocytes, which is induced by certain antigens in certain situations. This can be prolonged reversible, or fully irreversible.

Clonal deletion - Deletion of B-cells and T-cells that express self-antigens, this happens before they're immunocompetent. This is also called negative selection.

Clonal selection and expansion - These two go hand-in-hand. There are millions of different cells, each with different receptors that are specific for a particular antigen epitope. Antigen is then introduced to your body, and is presented to T or B cells. The cells that respond to the antigen are then selected, and cytokines then induce differentiation of that clone, massively increasing the number of specific antibodies against that particular pathogen.

This is the longer explanation:
According to the clonal selection theory, an animal first randomly generates a n enormous diversity of lymphocytes, and then selects for activation of those lymphocytes that can react against the foreign antigens that the animal actually encounters. As each lymphocytes develops in a central lymphoid organ, it becomes committed to react with a particular antigen. When a lymphocyte encounters its antigen in a peripheral lymphoid organ, the binding of the antigen to the receptors activates the lymphocyte, causing it to proliferate, thereby producing many more cells with the same receptor (clonal expansion [as cells derived from a common ancestor cell are referred to as a clone]). The encounter with antigen also causes the cells to differentiate into effector cells. An antigen therefore selectively stimulates those cells that express complementary antigen-specific receptors and are thus already committed to respond to it. And this is what makes our immune system so antigen-specific.

This provides a very useful framework for understanding the cellular basis of our immunological memory. The peripheral lymphoid organs contain a mixture of lymphocytes in at least three stages of maturation: na´ve cells, effector cells, and memory cells. When the na´ve cells encounter their antigen for the first time, the antigen stimulates some of them to proliferate and differentiate into effector cells, which then carry out an immne response (effector B cells secrete antibodies, whereas effector T cells either kill infected cells or influence the response of other cells). Some of the antigen-stimulated na´ve cells multiply and differentiate into memory cells, they are more easily and more quickly induced to become effector cells by a later encounter with the same antigen (but they do not carry out any immune response themselves.) When they encounter their antigen, the memory cells give rise to either effector cells or more memory cells.

And this is how the primary response generates immunological memory due to clonal expansion, whereby the proliferation of antigen-stimulated na´ve cells creates many memory cells, as well as because these memory cells are able to respond more sensitively, rapidly, and effectively to the same antigen than do na´ve cells. Also these memory cells may even persist in the body for the remaining lifetime of the patient, even in the absence of their specific antigen.
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