Food allergies occur when the immune system identifies the proteins in a particular food as harmful to the body, subsequently reacting to try to "fight off" the "harmful" proteins (which are in fact harmless). Allergy to green pepper (capsicum) is fairly rare. In fact, some people take capsicum supplements to help treat allergies.
Whereas most food allergens cause itching or swelling of the mouth, tongue or lips, more common allergic reactions to green pepper are respiratory symptoms, such as rhinoconjinctivitis (red, itchy, swollen, watery eyes) and asthma. Headaches are also not often reported in association with food allergies but are in the case of green pepper allergy. Other symptoms you may experience if you have a green pepper allergy include eczema, hives, swelling and redness of the extremities or face, abdominal pain, nausea, vomiting, diarrhea, runny nose, congestion, coughing, sneezing, wheezing, difficulty breathing, drop in blood pressure, lightheadedness or fainting.
You may experience some of these symptoms if you have oral allergy syndrome (OAS). This is an allergy to certain raw fruits and vegetables that is typically brought on by hay fever. If you have OAS it is likely that you will be able to eat cooked green peppers with no problem; you'll just have to avoid raw ones. If you have a true green pepper allergy you will have to avoid eating green peppers and may not even be able to handle them without experiencing symptoms. You should see an allergist to be tested and find out whether you have a green pepper allergy or OAS.
If you have a severe allergy to green pepper you may experience anaphylaxis, which can be fatal. Symptoms include nausea, vomiting, weak or rapid pulse, difficulty breathing, confusion and loss of consciousness. If you experience these symptoms, seek immediate medical attention.