T Helper Cells

T helper cells (also called helper T cells) are

Types of Helper T Cells

There are two distinct kinds:

Origin of Helper T Cells

Like all T cells, Th cells arise in the thymus.
Link to drawing showing the anatomy of the lymphatic system, including the location of the thymus. (52K).
The antigen-presenting cells (APCs) are called dendritic cells (DC). They
Discussion of how antigens are presented to T cells

There are two kinds of dendritic cell:

Th1 Cells

Th1 cells are produced when The major lymphokines secreted by Th1 cells are These

Th2 Cells

Th2 cells are produced when DC2-type dendritic cells present antigen to the T cell's receptor for antigen (TCR) and, presumably, one or more paracrine stimulants. The identity of the lymphokine(s) is still uncertain (indicated by a ? in the figure).

The major lymphokines secreted by Th2 cells are
Link to graphic showing how Th2 cells stimulate B cells to mature into antibody-secreting plasma cells.

Reciprocal inhibition of Th1 and Th2 cells.

The antigenic stimulus that sends pre-Th cells down one path or the other also sets the stage for reinforcing the response.

A Th1 response inhibits the Th2 path in two ways: A Th2 response inhibits the Th1 path:

Negative feedback of Th1 and Th2 cell formation

There is also evidence that late in the immune response, negative feedback mechanisms come into play to dampen the response.

Th1 and Th2 cells have different chemokine receptors.

Chemokines are cytokines that are chemotactic for (attract) leukocytes. Because they are chemotactic cytokines, chemokines are designated by the initials CC.

Chemokines bind to receptors on the responding leukocyte. The receptors are transmembrane proteins with the chemokine binding site exposed at the surface of the plasma membrane. Chemokine receptors are designated CCR.

With their different functions, we might expect that Th1 cells and Th2 cells would respond differently to chemokines. And so they do.

CCR3

The chemokine that binds to CCR3 is called eotaxin. It is secreted by epithelial cells and phagocytic cells in regions where allergic reactions are occurring. CCR3 is found on all cells implicated in allergic responses (e.g., asthma).

CCR5

CCR5 is found on CCR5 also acts - along with the CD4 molecule - as a receptor for HIV-1, the retrovirus that causes AIDS. This fact may explain
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24 June 1999