Normally, T cells travel throughout the body to detect and fight off foreign substances, such as viruses or bacteria.

CNN: Psoriasis

Another theory is that the initial illness may have created an autoimmune reaction, causing the body's immune system to attack normal cells as if they were foreign substances.

CNN: Post-polio syndrome

One theory to explain why early introduction is important holds that if babies aren't exposed early enough to certain foods, their immune systems will treat them as foreign substances and attack them, resulting in an allergy.

WSJ: Food Allergy Advice for Parents: Start Peanuts, Eggs and More Sooner