Some people do become allergic to cannabis, it can be directly by touching the plant or internal.
Antibody formation to THC has been demonstrated in animals. There are also clinical reports in humans that cannabinoids can exacerbate existing allergies, and there are several reports of demonstrated allergy to cannabinoids in humans (e.g. Freeman, 1983). Munson and Fehr (1983) concluded that: "it appears that cannabinoids can elicit the formation of specific antibodies ... [and that THC] or a metabolite is probably acting as a hapten, combining with a protein to form an antigenic complex" (p289). Hollister (1992), however, has questioned the clinical significance of
this evidence, arguing that:
While it is possible that a few persons may become truly allergic to cannabinoids, it is far more likely that allergic reactions, which have been extremely rare following the use of marijuana, are due to contaminants .. (e.g. bacteria, fungi, molds, parasites, worms, chemicals) that may be found in such field plants. That such impure material, when smoked and inhaled into the lungs, causes so little trouble is really a marvel (p163).
Smoking any organic material is an irritant, antihistamines might provide short-term relief
People have reported anaphylactic shock..meaning the throat can close up and unable to breath, and also hives.
If you use an inhaler (salbutamol sulphate), may aliviate the swelling, or a vaporizer.
Also may break out into hives after touching the weed-allergy to the plant.
Fertilizers and pecticides on the weed can also bring allergies as well as whatever it has been mixed with.
http://www.bioedonline.org/news/news.cfm?art=3369
http://www.sciencemag.org/cgi/content/short/316/5830/1494
http://www.health.gov.au/internet/main/publishing.nsf/Content/health-pubs-drug-cannab2-ch62.htm
http://www.steadyhealth.com/Allergic_To_Marijuana_t68387.html
This^^gives some advise as to how to test for allergies in weed.