Think of it like a strong bee sting. Be warned, if you are allergic to bees, you could have the same allergic reaction. A generalized envenomation is usually non-fatal and rarely severe. If you are envenomated, immidiately run your hand under the hottest water you can stand for as long as you can stand. This will help break down the proteins and help reduce the pain and the severity of the envenomation.
Here is an excerpt of an online article found here -
http://ozreef.org/library/articles/hazards_of_owning_a_reef.html
Abstract
Rabbitfish
Rabbitfish (genus Siganus) fins contain multiple spiny processes, all associated with venom glands. To be exact, there are 13 dorsal, 4 pelvic, and 7 anal spines. The spines have two deep grooves that contain venom glands along the distal third of the spine. The spine is covered with an epidermal layer that is ruptured by trauma, releasing the venom in much the same way as with stingrays. The pain is sudden and severe lasting for hours to several days and producing considerable distress. Like surgeonfish, the wound swells, becomes numb and pain radiates to the regional lymph nodes. The immediate area can become cyanotic with a surrounding red area.
Anaphylaxis
Anaphylaxis, an allergic reaction to the animal's venom, is a medical emergency. Anaphylaxis is a systemic immune response to a compound. It is independent of the compound's innate toxicity. Because it is dependent upon the immune system, an anaphylactic response requires prior exposure to the compound. Following this initial exposure, there will be no physically evident symptoms. One could consider this exposure as "priming" the immune system. Later contact, either the next time or several times down the road, will produce a systemic, life-threatening response in susceptible individuals within seconds to minutes of re-exposure. Again, this response is variable from person to person, and generally independent of the level of exposure. This reaction is characterized by vascular collapse and shock, with or without respiratory distress. Possible symptoms include:
Upper airway swelling -- hoarseness and a "lump in the throat" feeling.
Lower airway obstruction -- chest tightness.
Wheezing, and shortness of breath.
Cutaneus involvement -- intensely itchy, red, raised hives with pale centers and wandering borders, that can be local or spread over the entire body.
Gastrointestinal symptoms -- nausea, vomiting, diarrhea, and cramping stomach pains.
Vascular collapse -- blood pressure drop, rapid heart rate, and shock.
Anaphylaxis requires prompt medical treatment with epinephrine and other support measures. Anyone who has experienced a severe allergic reaction should avoid exposure to the precipitating compound. If this compound is a toxin produced by a reef inhabitant, the animal should be removed from the aquarium. At a minimum, epinephrine should be readily available for immediate use in case the individual is exposed to the compound again. Because anaphylaxis is immune regulated, each response will likely be more severe than any previous exposure.