My penis has an alergic reaction and is swollen?

See Your Doctor. Swelling in the penis can be dangerous and needs treatment. It may or may not be an allergic reaction. Allergic reactions in an isolated area are usually the result of a contact allergen. It could also be an infection or a constriction that could cause the swelling.
Other possibilities. A swollen glans (tip of penis) could also be due to balanitis (inflammation of the glans) related to fungal or bacterial infection, or other causes. It is important to be seen by a medical professional for proper diagnosis, as treatment can prevent complications (eg phimosis; a narrowing of the foreskin). Also, if fungal, evaluation for diabetes is worthwhile.
Penis swelling. If you feel pretty sure that it is an allergic reaction, the first step is to take an antihistamine like Benadryl (diphenhydramine) (otc). If the swelling is severe or painful, see a doctor; a prescription for cortisone may help. Allergic swelling usually itches, but does not hurt. If there is significant pain, then other causes need to be considered.