HIV has a unique ability to evade the immune system of the host. By evading the immune system HIV gets the first foothold on a host (human) and evades the control and elimination of infection by the host. There are several mechanisms by which HIV evades the host immune system.
After getting a foothold in a human host, HIV establishes a sustained level of replication of the virus. The new generations of HIV also evade the host immune system by mutation and recombination and maintain viral diversity.
Due to the very high rate of virus replication as well as continuous mutation (responsible for vaccine ineffectiveness), the neutralizing antibodies that are formed by host defense can not neutralize the rapidly replicating and continuously mutating HIV.
The envelope proteins gp120 and gp41 are the main targets of neutralizing antibodies against HIV. But these neutralizing antibodies are evaded by HIV, by increased glycosylation of the envelope, changing the primary sequence of the envelope, and masking neutralizing epitopes.
HIV infects mainly the CD4+ T cells (including HIV-specific CD4+ T cells), which are essential for humoral and cell-mediated immune responses. By infecting the CD4+ T cells, HIV eliminates the ability of the host to initiate an appropriate immune response against HIV. By controlling the HIV-specific CD4+ T cells HIV has a negative consequence on the host immune mechanism.
HIV-infected cells are sequestrated in immunologically privileged sites such as the central nervous system, which is another way of escaping or evading the host immune system. By escaping the primary elimination, HIV creates a pool of infected cells that can not be eliminated by even HIV-specific cytolytic T lymphocytes (CTLs).