The immune system carries out the body's defense against infections produced by pathogenic (disease-causing) agents and it also destroys or eliminates foreign bodies. It is now known that HIV erodes the immune system by killing the cells, which compose it. The more cells that die as a result of HIV infection, the more difficult the body's struggle against infection becomes.
The cells that HIV preferentially attacks are known as CD4 cells or T4 lymphocytes. They are the most important cells for the body's defense. A count of these cells in the blood also serves as an index for monitoring the infection's progress: as infection progresses, the number of these cells lowers.
In persons who have AIDS, infections that occur as a result of HIV infection are also known as "opportunistic" because they are contracted only when the immune system fails to function adequately and the ability to fight disease-causing agents has been lost.