Before discussing HIV, it’s helpful to talk a bit about the body’s immune system.
What is the Immune System? 
The immune system is the body’s mechanism for protecting itself against the many harmful organisms, such as bacteria and viruses, that try to invade it. It's a highly complex network of cells, tissues and organs, including blood cells.
How does the Immune System work?
When an organism gets inside your body, white blood cells that form part of your immune system go to investigate. These white blood cells come in two groups: one group organizes the other, which in turn produces the antibodies to fight the hostile organisms.
The cells that do the organizing are called T-cells, or often referred to as CD4 cells.
What is HIV?
HIV stands for Human Immunodeficiency Virus. Many people also refer to HIV as the "AIDS virus."
What does HIV do to the body?
HIV is dangerous because the virus destroys the very cells of our immune systems that are supposed to be keeping us well: the CD4 cells. HIV attaches itself to a CD4 cell and enters. It makes copies of itself inside the CD4 cell and then goes on to damage and destroy the cell. The new HIV viruses burst out of the CD4 cell and go off to find more cells to invade.
If the number of CD4 cells is reduced, the immune system has fewer cells to help it defend the body from other organisms. This means we are at greater risk of getting ill. The immune system does try to fight HIV infection. It produces antibodies to do this. But they’re not very effective without the CD4 cells to organize them.