Pamela Georgeson D.O., a member of the American Osteopathic Association, is a board-certified osteopathic physician in pediatrics, as well as in allergy and immunology. She gives the following answer:
Image: UNIVERSITY OF ARIZONA
Sneeze: to make a sudden violent spasmodic audible expiration of breath through the nose and mouth, especially as a reflex act.
The nose provides the main route through which inhaled air enters and leaves the lower airways. Because of its position, it serves numerous functions. The narrowing passageways cause inspired air to flow with increased turbulence. This turbulence in turn increases the interaction between the airstream and the nasal mucosa (lining of the nose), allowing for heat and moisture to be exchanged, and for suspended or soluble particles from the air to be cleared.
Sneezing is a physiologic response to the irritation of the respiratory epithelium lining of the nose. The process usually begins with the release of chemicals such as histamine or leukotrienes. These substances are manufactured by inflammatory cells such as eosinophils and mast cells typically found within the nasal mucosa. Chemical release is caused by viral respiratory infections, filtered particles, allergens (substances that trigger allergic reactions) or physical irritants such as smoke, pollution, perfumes and cold air. Allergic reactions with the nasal mucosa require the presence of IgE (allergy antibody specific for the allergen). This leads to fluid leakage from vessels in the nose, causing symptoms of congestion and nasal drip. Additionally, nerve endings are stimulated, leading to the sensation of itching.
Ultimately, the nerve ending stimulation leads to activation of a reflex inside the brain. The nervous impulse travels up the sensory nerves and down the nerves controlling muscles in the head and neck, and that leads to the rapid expulsion of air. The high velocity of the airflow is achieved by the buildup of pressure inside the chest with the vocal chords closed. Sudden opening of the cords allows the pressurized air to flow back up the respiratory tract to expel the irritants. This helps to remove offending particles in the nose. However, in infected individuals, it also allows for the spread of the common cold, as innumerable viral particles are contained within each droplet of mucus expelled.
Various medications are available to help control this reaction. Antihistamines act principally by blocking the action of histamine at receptors located on the blood vessels in the nose. Some prescription antihistamines are nonsedating, compared with their over-the-counter counterparts. Decongestants stimulate receptors located on the same vessels to cause constriction and lessen the congestion. Topical nasal steroids are used in allergic patients to reduce the number of inflammatory cells and ultimately inhibit the release of histamine.