The Immune System

The immune system is a system within all vertebrates (animals with a backbone) which in general terms, is comprised of two important cell types: the B-cell and the T-cell. The B-cell is responsible for the production of antibodies (proteins which can bind to specific molecular shapes), and the T-cell (two types) is responsible either for helping the B-cell to make antibodies, or for the killing of damaged or "different" cells (all foreign cells except bacteria) within the body. The two main types of T-cells are the "helper"T-cell and the cytotoxic T-cell. The T-helper population is further divided into those which help B-cells (Th2) and those which help cytotoxic T-cells (Th1). Therefore, in order for a B-cell to do its job requires the biochemical help of Th2 helper T-cells; and, for a cytotoxic T-cell to be able to eliminate a damaged cell (say, a virally-infected cell), requires the biochemical help of a Th1 helper T-cell.

Whenever any foreign substance or agent enters our body, the immune system is activated. Both B- and T-cell members respond to the threat, which eventually results in the elimination of the substance or agent from our bodies. If the agent which gains entry is the kind which remains outside of our cells all of the time (extracellular pathogen), or much of the time (virus often released) the "best" response is the production by B-cells of antibodies which circulate all around the body in the bloodstream, and eventually bind to the agent. There are mechanisms available which are very good at destroying anything which has an antibody bound to it. On the other hand, if the agent is one which goes inside one of our cells and remains there most of the time (intracellular pathogens like viruses or certain bacteria which require the inside of one of our cells in order to live), the "best" response is the activation of cytotoxic T-cells (circulate in the bloodstream and lymph), which eliminate the agent through killing of the cell which contains the agent (agent is otherwise "hidden"). Both of these kinds of responses (B-cell or cytotoxic T-cell) of course require specific helper T-cell biochemical information as described above.

Usually, both B-cell and cytotoxic T-cell responses occur against intracellular agents which provides a two-pronged attack. Normally, these actions are wonderfully protective of us. The effect of HIV on the immune system is the result of a gradual (usually) elimination of the Th1 and Th2 helper T-cell sub-populations.

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