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acquired immunity

Sometimes also called specific or adaptive immunity, acquired immunity is those features of the immune system that are "learned" during a person's lifetime rather than the ones the individual is born with. This is the part of the immune system that deals with specific invaders and learns to recognise them by exposure to them.

The other part of the immune system, the one that we are born with, is called the innate immune system and consists of many mechanisms which are all non-specific - that is they are not programmed to recognise any particular invaders.

Acquired immunity is further divided into two parts:

Both parts of the acquired immune system responds to peptide sequences called antigens. Antigens are sections of broken up proteins from cells that have been ingested by several different types of leukocyte. These leukocytes then present these antigens to both B- and T-cells and are known as antigen presenting cells (APC). Antigens are the way that the acquired immune system recognises invading bacteria, viruses and other harmful organisms (pathogens). Both B- and T-cells have surface receptors that recognise specific antigens. All the receptors on each individual lymphocyte are monoclonal which means that they are all the same and so each lymphocyte is programmed only to recognise a specific antigen.

Humoral Immunity

When a B-cell locates an antigen it doesn't react with it but divides repeatedly to produce identical daughter lymphocytes. Most of these B-cells then transform into plasma cells. These then shed their receptors in a soluble form known as antibodies and release them into the blood and lymph fluid. However, some of the activated B-cells do not become plasma cells but become what are known as memory B-cells which continue to produce small amounts of the antibody long after the infection has been overcome.

When an antibody binds to an antigen on a cell, it acts as a signal for neutrophils, eosinophils, basophils and macrophages to engulf and kill (phagocytose) it in a process known as opsinisation. Another thing that happens when antibodies bind to cells is that serum proteins known as the complement bind to the immobilized antibodies and destroy the attached cells. Antibodies also signal natural killer cells and macrophages to kill cells that are internally infected with viruses and bacteria. Antibodies will also neutralise many pathogens simply by binding to them.

Cell Mediated Immunity

T-cells are manufactured in the thymus where they are tested for their response to self-tissue. Any that respond to self-tissue are usually rejected. However, it is clear that in T-cell mediated autoimmune diseases (e.g. multiple sclerosis) some T-cells with self-receptors somehow get manufactured.

There are three main types of T-Cell - Helper T-cells, Killer T-cells and Memory T-cells and all three recognise specific antigens.

When a helper T-cell recognises its antigen it migrates to the secondary lymphoid tissue where it divides into multiple memory T-cells and killer T-cells. It is the killer T-cells that actually destroys any cells it meets that express the antigen that its receptors match for.

Acquired Immunity Links:
Cell-mediated Immunity
Humoral Immunity
Interrelationship between innate & acquired immunity
A movie of a killer T-cell in action

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