How could HIV infect the mouth and throat?

At least one test-tube study¹ has shown that cells in the lining of the mouth (known as normal human oral keratinocytes) can become infected when exposed to high concentrations of HIV, and can pass this infection on to lymphocytes. The tonsils also appear to be susceptible to infection,² and infectious HIV has been found in high levels in the tissues of the pharynx and the tonsils.

However, while the vaginal and rectal tissues contain many cells vulnerable to HIV infection (possessing receptors which enable HIV to bind onto them), few such cells are present in the mucous membrane of the mouth and throat. The tissue of the mouth and oesophagus is also very thick compared with genital tissues, and fluids stay in contact with it for a very short time because swallowing clears the mouth regularly. The mouth is therefore generally regarded as an unlikely route of HIV transmission.

There is also strong evidence that an enzyme in saliva called secretory leukocyte protease inhibitor (SLPI) prevents HIV infection of lymphocytes, and therefore inhibits HIV infection in the mouth and throat. HIV antibodies have also been found in saliva, but it is not clear how much of a protective role they play.^{3 4} A 1989 study showed that even when greatly diluted, saliva was still capable of preventing HIV infection of white blood cells. It is still unclear how this happens and which specific substance in saliva inhibits infection.⁵

There is no evidence to either prove or disprove whether HIV is destroyed in the stomach. Researchers assume that, like other viruses, HIV will be inactivated by the digestive enzymes and acidity of the stomach. It is not possible to say whether semen that enters the stomach might enter the bloodstream through stomach ulcers or other inflamed tissue, but it seems very likely that HIV transmission through the stomach is much less of a possibility than transmission through the mouth and throat.