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I mean, everyone knows that AIDS patients don't die of the HIV infection, rather the opportunistic infections. But if HIV only affects T lymphocytes, and destroys them, then that means only cell mediated immunity and humoral immunities are affected (since B cells need helper T cells to be activated).

The body should still be producing other types of leukocytes like neutrophils and macrophages, which should eliminate most bacteria. So does HIV infection render the mononucleaf phagocytic system useless as well? If yes, how so?

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Clinical diagnosis of AIDS in a person infected with HIV is based on the following criteria:

  • A CD4+ T-cell count below 200 cells/µl (or a CD4+ T-cell percentage of total lymphocytes of less than 14%)

OR

  • The patients has one of the few "indicatory" diseases that strongly suggest severe immunodeficiency. See the list of this diseases here. Usually they have fatal consequences.

As we can observe, there are mostly fungal and viral infections that live inside the host cells or (in case of tuberculosis) diseases caused by intracellular bacteria. They can be overcame only with working mechanisms of cellular immunity. Other mechanisms are simply too slow, not specific or not precise enough.

Another issue is an impairment of the immunological surveillance and much higher risk of cancer in individuals diagnosed with AIDS. Precise elimination of malignant cells is fully dependent on cellular immunity.

[EDITED] For further reading, recent paper analysing the most common causes of death in patients with AIDS in the era of anti-retroviral therapy.

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First, without support and amplification from the adaptive immune system, the innate response is less efficient.

Second, virtually every pathogen, whether obligate or opportunistic, has mechanisms that allow it to evade or subvert the innate immune response. That's why AIDS patients are susceptible to pathogens (including opportunistic pathogens) but are still not infected with non-pathogens, such as environmental agents or pathogens of, say, plants or insects even though we're constantly soaking in those non-pathogen microorganisms. Those agents don't have the needed ability to avoid innate responses.

So even in AIDS patients, innate immune responses are helpful, it's just that they're not enough, on their own, to prevent infections by pathogens with ways of evading innate immunity.

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