Question

Why is a retrovirus like HIV almost invariably capable of evading the host's immune system when many other pathogens are not?

Answer 1

Antiviral responses of the immune system usually induce type I interferon (IFN) recognizing viral nucleic acids. These antiviral responses inhibit viral replication by different mechanisms. They also enhance the ability of APCs and help in viral clearance and immunological memory.

HIV-1 and other retroviruses do not directly alert host innate defenses to their presence, and HIV-1 does not induce IFN in the CD4+ T lymphocytes but affected individuals suffer from loss of CD4+ T cells with elevated cytokine and immunoglobulin levels (immune activation). The high level of circulating type I IFN in AIDS patients is due to the breakdown of mucosal barrier and bacterial particle entry into the circulation and other such mechanisms. The failure of the innate immune response to directly recognize the virus is the starting point at which HIV evades the immune system. The proteins on the viral surface mutate rapidly, which becomes difficult for the antibodies to bind with the virus and once the virus affects the immune system and replicates, the host's immune system fails.