Note: this site last updated in 2006
helper T cell
An article from "The Molecules of HIV" (c) Dan Stowell
The helper T lymphocyte has been termed "the leader of the immunological orchestra". Helper T cells circulate throughout the body and check for antigens (substances that might be signs of invading virus/bacteria/etc). (The mechanism of checking is actually that they interface with HLA%20class%20I">HLA class II protein on other cells, checking what kind of substance HLA%20class%20I">HLA class II is presenting.)
If the helper T cell is stimulated by contact with antigen, it responds by cell division, as well as producing lymphokines and chemokines">chemokines. Lymphokines">Lymphokines and chemokines">chemokines are chemical messages which alert other cells that there is an antigen to be dealt with; cell division means that there are more and more activated helper T cells around to sound the alarm.
In the following diagram you can see the helper T cells (marked TH) at the centre of the immune response:
The helper T cell is normally the main cell which carries the CD4 surface protein. This means that it's the main type of cell targeted by HIV. (Not all helper T cells are CD4+ cells - a small minority of them carry CD8 instead.) The above description of the helper T cell's role should tell you the reason that HIV is so crippling to the immune system: it can destroy the majority of CD4+ cells in the human immune system - and the orchestra, having no leader, can't do very much. The body becomes immunocompromised.