As we inhale allergens, they accumulate in our bodies, are seen as the foreign substances that they are, are thus attacked by immune cells, and eventually an immune reaction is triggered. Antibodies are produced, which generate histamines and other chemicals to fight the battle. Histamines cause the blood vessels and tissues in your nasal passages and sinuses to swell, become inflamed, and produce excess mucus. While many experience this assault from the chest up, others have a whole-body reaction, including joint pain and stiffness and general malaise. As the body huffs and puffs to exterminate these unwelcome visitors, severe fatigue can set in.