The body's first line of defense against invaders includes the nose, mouth, eyes, lungs, and stomach. When the immune system reacts to an allergen, these body parts become battlegrounds.
Signs of the battle can include one or more of the following: runny nose; sneezing; watery, swollen, or red eyes; nasal congestion; sinus inflammation and pressure; hives; rashes; itchy eyes; itchy nose; wheezing; shortness of breath; a tight feeling in the chest; difficulty breathing; coughing; diarrhea; nausea; headache; fatigue; and a general feeling of misery.
It's ironic that the immune system, designed to protect you from illness, produces symptoms that make you feel sick when it overreacts to mundane substances. But that's the nature of the allergic response. The symptoms are the unfortunate result of the immune system's overperformance. It's a perfect example of the old saying that sometimes the cure is worse than the disease.
The source of allergies is not a mysterious one. Simply put, it is the rejection of a non-dangerous substance by your body's immune system.