It is quite possible that you could have an allergic reaction (anaphylactic shock) to even a small amount of venom from a harmless tarantula, much as some people react badly to a bee sting. An allergic reaction assumedly could include a rash, itching, shortness of breath, and collapse. Anaphylactic shock can be treated in familiar ways in any emergency room, but again I’m not aware any cases in theUnited States.
You are much more likely to develop localized and sometimes dangerous rashes from the urticating hairs on the abdomens of many common American tarantulas. The species of Aphonopelma and Brachypelma, among others, are notorious for kicking thick clouds of bristles off their abdomens with a hind leg, and the webs of Avicularia species are notoriously bristle-laden.
Pseudotheraphosa spp. and Theraphosa spp. hairs can cause extensive bleeding sores on humans. The cages of any of these tarantulas may be coated inside with urticating hairs, which could be breathed in each time you open the cage. Then the bristles can work their way into your nose, lips, and mouth, where they cause irritation to mucous tissues with swelling and redness. If a tarantula sprays you with bristles, many could become embedded in the tissues around your eye (or even in the eye itself), causing painful swelling and even temporary blindness until the eye is cleaned by a doctor. A child holding a tarantula next to the face could be in serious, though temporary, trouble. Hand-washing is a must after handling tarantulas and their cages.
There are many anecdotal reports of keepers developing allergies to urticating hairs and being forced to give up their tarantulas or face repeated hospital treatment, and some perhaps are true. There is little doubt that irritation from these bristles can be painful and annoying, and if present in large numbers, the bristles could cause serious eye and throat problems (possibly blocking the air passages or causing blindness).
It seems that the bristles do not carry any toxins, so the damage they cause is strictly physical, a result of the barbs having worked their way into the tissues; this means that true allergic reactions are not likely, though certainly not impossible. Rashes on the hands and arms are commonly treated with hydrocortisone creams, whereas affected eyes may need special rinses under the guidance of a doctor. Urticating hairs are not something to ignore if you believe you could have stronger than normal reactions to everyday activities involving your tarantula.