What Happens to Antigens?

Antibodies

The antibodies that B cells produce are basic templates with a special region that is highly specific to target a given antigen. Much like a car coming off a production line, the antibody's frame remains constant, but through chemical and cellular messages, the immune system selects a green sedan, a red convertible, or a white truck to combat this particular invader.

However, in contrast to cars, the variety of antibodies is very large. Different antibodies are destined for different purposes. Some coat the foreign invaders to make them attractive to the circulating scavenger cells, phagocytes, that will engulf an unwelcome microbe.

When some antibodies combine with antigens, they activate a cascade of nine proteins, known as complement, that have been circulating in inactive form in the blood. Complement forms a partnership with antibodies, once they have reacted with antigen, to help destroy foreign invaders and remove them from the body. Still other types of antibodies block viruses from entering cells.

T Cells
T cells have two major roles in immune defense. Regulatory T cells are essential for orchestrating the response of an elaborate system of different types of immune cells.

Helper T cells, for example, also known as CD4 positive T cells (CD4+ T cells), alert B cells to start making antibodies. They also can activate other T cells and immune system scavenger cells called macrophages and influence which type of antibody is produced.

Certain T cells, called CD8 positive T cells (CD8+ T cells), can become killer cells that attack and destroy infected cells. The killer T cells are also called cytotoxic T cells or CTLs (cytotoxic lymphocytes).

T lymphocytes become CD4+, or helper T cells, or they can become CD8+ cells, which in turn can become killer T cells, also called cytotoxic T cells.

 

Immune System Process
Activation of Helper T Cells

 

 

After it engulfs and processes an antigen, the macrophage displays the antigen fragments combined with a Class II MHC protein on the macrophage cell surface. The antigen-protein combination attracts a helper T cell, and promotes its activation.

Activation of Cytotoxic T Cells

After a macrophage engulfs and processes an antigen, the macrophage displays the antigen fragments combined with a Class I MHC protein on the macrophage cell surface. A receptor on a circulating, resting cytotoxic T cell recognizes the antigen-protein complex and binds to it. The binding process and a helper T cell activate the cytotoxic T cell so that it can attack and destroy the diseased cell.

Activation of B Cells to Make Antibody

A "B" cell uses one of its receptors to bind to its matching antigen, which the B cell engulfs and processes. The "B" cell then displays a piece of the antigen, bound to a Class II MHC protein, on the cell surface. This whole complex then binds to an activated helper T cell. This binding process stimulates the transformation of the "B" cell into an antibody-secreting plasma cell.


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