T-cells and antibodies are different arms of the immune response. T cells actually exist in several sub-types, but are involved in the cell mediated immunity part of the immune system. This means some sub-types of T cells, such as cytotoxic T cells destroy infected cells, and other types of T cell such as helper T cells activate various other cells which engulf and destroy infected cells and invading pathogens such as bacteria. Antibodies on the other hand, form the part of the humoral immue response (also called antibody mediated immunity). This is a different arm which works in conjunction with the cell mediated response described above. This arm is comprised of B cells which become activated when they encounter part of an invading pathogen. This may happen because macrophages (one of the types of cells that engulf and destroy invading pathogens), after destroying e.g. a bacteria, displays part of the bacteria on its membrane, alerting the B cells that foriegn invaders are present. Alternatively, the B cells may be activated by the previously mentioned T cells, as the two arms of the immune system work together. Once the B cells are activated, they begin to produce antbodies. These antibodies can latch onto viruses, making it harder for them to infect other cells (neutralisation), or they can clump up lots of pathogens together (agglutination), so parts of the cell mediated immunity can destroy them more easily. So the antibodies and the T cells are different things (antibodies aren't actually full cells themselves) and form different aprts of the immune system that work together to help defeat infections. The problem with understanding the immune system, is that A) it's very complicated when you study it in detial and B) there are many different parts to it and names such as white blood cells are vague and confusing when you have to study it in more detail. The unusual thing about HIV is that rather than infecting, for example, stomach cells, it likes to infect helper T cells (also called CD4+ T cells) of the immune system, the very system that is trying to stop infection. So, as the infection spreads, more and more helper T cells are infected and so more and more are identified as being needed to be destroyed. As more and more are destroyed, the CD4 cell count goes down, which can be used in a blood test to measure how far the infection has progressed. The ELISA test you mention works in a different way. Antobodies to HIV are specific only to HIV, so they won't be present if someone has, say, the flu instead (unless the person has HIV and the flu at the same time of course). So, measuring HIV antibodies gives another indicator of how prolific the infection is. In order to do this, a blood sample is taken, which may or may not contain HIV antibodies which can bind to HIV. This sample is then treated with a different type of antibodies, which is specially produced to bind to the antibody against HIV. This secondary antibody is also bound to an enzyme which allows a reaction to occur, such as changing the colour of a solution. The sample is than washed to remove anything that is not bound. So, if the person is not infected, there will be no HIV antibody present, so the secondary antibody cannot bind to anything, so when the sample is washed, the secondary antibody and the enzyme are both washed away, and the colour change reaction cannot occur. If the person is infected, there should be HIV antibody present. This means the secondary antibody will bind to the HIV antibody, and the enzyme on the secondary antibody will not be washed away, so that it can allow the colour change reaction to ccur, and the solution will change colour, indicating the HIV antibody is present. I hope this helps, any questions about it, feel free to post a comment. I know it can be quite difficult to understand.