Sneezing is a physiological response to expel irritants, such as viruses, smoke, peppers and pollens, from our respiratory systems. When these irritants are detected, the body releases chemical called histamine, which stimulates nerve-endings and causes the itching sensation. A reflex inside the brain is then activated, causing the throat muscles to constrict and build up air pressure within the chest. Next, the chest muscles contract vigorously while the throat muscles are suddenly relaxed, causing the pressurized air to flow back up the respiratory tract and expel the irritants.