Sometimes a person will have a reaction to an insect sting that is more than the normal sting reaction but may not be an allergic reaction.

• Toxic reaction: A toxic reaction can cause symptoms similar to those of an allergic reaction, including nausea, fever, swelling at the site of the sting, fainting, seizures, shock, and even death. A toxic reaction occurs when the insect venom acts like a poison in the body and may result after only one sting, but it usually takes many stings from insects that are not normally considered poisonous. Spider venom usually causes a toxic reaction.
• Serum sickness: Serum sickness is an unusual reaction to a foreign substance in the body. You can have a delayed reaction to an insect sting that causes serum sickness hours or days after the sting. Symptoms include fever, joint pain, other flu-like symptoms, and sometimes hives. Serum sickness may also be due to an immune system reaction to the sting. If you develop serum sickness, you may be at greater risk of developing a severe allergic reaction to future stings.

Less commonly, bites from certain insects—such as deerflies, horseflies, kissing bugs, bedbugs, and mosquitoes—can cause allergic reactions. But the reaction from bites is usually more mild than from stings.