\(HIV\) that causes \(AIDS\), first starts destroying

(a) : The \(AIDS\) retrovirus, called human immunodeficiency virus \((HIV)\), mounts a direct attack on \(CD4^+\ T\) helper cells because it recognizes the \(CD4\) coreceptors associated with these cells. \(HIV’s\) attack on \(CD4^+\ T\) cells cripples the immune system in at least three ways. First, HIV­infected cells die only after releasing replicated viruses that infect other \(CD4^+\ T\) cells, until the entire population of \(CD4^+\ T\) cell is destroyed. Second, \(HIV\) causes infected \(CD4^+ \ T\) 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 \(CD4^+\ T\) 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.