Allergies are caused by a reaction of the immune system to harmless antigens (allergens) e.g. house dust, pollen.
Particles are inhaled into lungs.
Recognised by immune system as foreign.
B cells produce IgE antibodies.
Antibodies coat surface of mast cells in airways.
Body is now sensitised to the allergen.
When allergen enters body again, it binds to antibody molecules on mast cells and stimulates release of histamine.
Histamine causes blood vessels to widen and become leaky. Fluid and white blood cells leave capillaries and area becomes hot and inflamed.
Asthma sufferers also secrete loads of mucus and airways become blocked with fluid. Smooth muscles round trachea, bronchi and bronchiole contract, further narrowing airways. Breathing is difficult and wheezing, coughing and shortness of breath is common.
There is no cure for asthma - use inhalers. Vaccines being developed to make response less severe by desensitising people so they don't make antibodies to allergens. (Asthma partly inherited, on increase because of pollution problems and passive smoking)