They are an important part of the immune system. There are two main types of T-cells. They lead the attack against infections.
Researchers can tell the T-cells apart by specific proteins on the cell surface.
This is a sign that the immune system is being weakened. The lower the T-cell count, the more likely the person will get sick. There are millions of different families of T-cells. Each family is designed to fight a specific type of germ. When HIV reduces the number of T-cells, some of these families can be totally wiped out.