Q.Why are allergies common among people living in metropolitan cities?

Vaccination (immunization) works by deliberately exposing the body to a harmless or weakened form of an antigen — a killed or attenuated pathogen, or one of its components — without causing the actual disease. This exposure still triggers a real primary immune response, generating memory B-cells and memory T-cells specific to that antigen. If the person later encounters the real, disease-causing pathogen, these memory cells allow a much faster and stronger secondary immune response, which typically clears the pathogen before it can cause illness.

When the immune system itself malfunctions, the resulting immunological disorders are usually grouped into three broad types. Immunodeficiency disorders occur when the immune system fails to mount an adequate response — whether present from birth (primary) or acquired later in life (secondary) — leaving the person unusually vulnerable to infections. Hypersensitivity (allergic) disorders are an exaggerated, harmful immune reaction to a normally harmless substance (an allergen); allergies are notably more common among people living in crowded metropolitan cities, likely because of greater everyday exposure to air pollutants, dust and other allergens in a dense urban environment. Autoimmune disorders arise when the immune system mistakenly treats the body's own healthy tissue as if it were foreign and attacks it — examples include Graves' disease, in which the immune system overstimulates the thyroid gland, and rheumato …