The CD4 Test
HIV attacks a type of immune system cell called the T-helper cell. This cell carries on its surface a protein called CD4, which HIV uses to attach itself to the cell before gaining entry.

The T-helper cell plays an important part in the immune system by helping to co-ordinate all the other cells to fight illnesses. A major reduction in the number of T-helper cells can have a serious effect on the immune system.

HIV causes many T-helper cells to be damaged or destroyed. As a result, there are fewer cells available to help the immune system to fight illnesses.

The CD4 test measures the number of T-helper cells in your blood. The more cells you have per cubic millimetre of blood, the stronger is your immune system. The stronger your immune system, the better your body can fight illnesses. A low CD4 count does not mean that you will certainly become ill, but it makes it more likely.

HIV, our Immune System and Antibodies
Antigens are large molecules (usually proteins) on the surface of cells, viruses, fungi, bacteria, and some non-living substances such as toxins, chemicals, drugs, and foreign particles. The immune system recognizes antigens and produces antibodies that destroy substances containing antigens. After getting the HIV infection, our immune system starts producing antibodies to HIV. These antibodies are the source for testing the HIV infection.

The dangerous "window period"
If you become infected with HIV, it usually takes between three weeks and two months for your immune system to produce antibodies to HIV. This period may extend upto 6 months in some cases(5%). If you think you were exposed to HIV, you should wait at least for two months before being tested. You can also test right away and then again after two or three months. During this "window period" an antibody test may give a negative result, but you can transmit the virus to others if you are infected.

Antiretroviral therapy during the window period can delay the formation of antibodies and extend the window period beyond 12 months.

Different Types of HIV Tests
Antibody test results for HIV are accurate more than 99.5% of the time. Before you get the results, the test has usually been done two or more times. The first test is called an "EIA" or "ELISA" test. Before a positive ELISA test result is reported, it is confirmed by another test called a "Western Blot."

ELISA
The ELISA test, or the enzyme immunoassay (EIA), was the first screening test commonly employed. It has a high sensitivity.

The test proceeds by the general ELISA method: the person's serum is diluted 400 fold and applied to a plate to which HIV antigens have been attached. Some of the antibodies in the serum may bind to these HIV antigens. The plate is then washed to remove all other components of the serum. Then a specially prepared "secondary antibody"—an antibody that binds to human antibodies—is applied to the plate, followed by washes. This secondary antibody is chemically linked in advance to an enzyme . Thus the plate will contain enzyme in proportion to the amount of secondary antibody bound to the plate. A substrate for the enzyme is applied, and catalysis by the enzyme leads to a change in color or fluorescence. As the ELISA results are reported as a number, the most controversial aspect of this test is deciding the "cut off" point between positive and negative.

Western blot
The Western blot test uses the general Western blot procedure. HIV-infected cells are opened and the contained proteins are entered into a slab of gel to which a voltage is applied. Different proteins will move with different velocities in this field, depending on their size, while their electrical charge is leveled by a substance, called sodium lauryl sulfate. Once the proteins are well separated, they are transferred to a membrane and the procedure continues similar to ELISA: the person's diluted serum is applied to the membrane and antibodies in the serum may attach to some of the HIV proteins. Antibodies which do not attach are washed away, and enzyme-linked antibodies with the capability to attach to the person's antibodies first detect to which HIV proteins the person has antibodies.