(3)

HIV enters into helper T-lymphocytes (${\mathrm{T}}_{\mathrm{H}}$), replicates and produces progeny viruses. The progeny viruses released in the blood attack other helper T-lymphocytes. This is repeated, leading to a progressive decrease in number of helper T-lymphocytes in the body of infected person.

(1)

The AIDS retrovirus, called human immunodeficiency virus (HIV), mounts a direct attack on ${\mathrm{CD}}_4^{+}$ T helper cells because it recognizes the ${\mathrm{CD}}_4$ coreceptors associated with these cells.
HIV attack on ${\mathrm{CD}}_4^{+}$ T cells cripples the immune system in at least three ways. First, HIV-infected cells die only after releasing replicated viruses that infect other ${\mathrm{CD}}_4^{+}$ T cells, until the entire population of ${\mathrm{CD}}_4^{+}$ T cells is destroyed. Second, HIV causes infected ${\mathrm{CD}}_4^{+}$ T cells to secrete a soluble suppressing factor that blocks other T cells from responding to the HIV antigen. Finally, HIV may block transcription of MHC genes, hindering the recognition and destruction of infected ${\mathrm{CD}}_4^{+}$ T cells, and thus protecting those cells from any remaining vestiges of the immune system. The combined effect of these responses to HIV infection is to wipe out the human immune defense.

(3)

AIDS is a disorder of cell-mediated immune system of the body. Virus responsible for AIDS is HIV (Human Immunodeficiency Virus). There is a reduction in the number of helper T-cells which stimulate antibody production by B-cells. This results in the loss of natural defence against viral infection.