Human Immunodeficiency Virus attacks the CD4 immune cells, which are the immune system cells. CD4 cells trigger the immune system to create antibodies in order to fight off infection. As HIV destroys the CD4 cells, the function of immune system is suppressed and the affected person is not able to defend infection. As HIV reduces the CD4 cells, in your body, the person is unable to defend from infection. The immunosuppressed patient develops illness, infection and ultimately, AIDS.