The immune system is the body's natural defense system that helps
fight infections. The immune system is made up of antibodies, white blood
cells, and other chemicals and proteins that attack and destroy substances such
as bacteria and viruses that they recognize as foreign and different from the
body's normal healthy tissues.
The immune system is also responsible for allergic reactions and
allergies, which may occur when the immune system incorrectly identifies a
substance (allergen), such as pollen, mold, chemicals, plants, and medicines,
as harmful.
Sometimes the immune system also mistakenly attacks the body's own
cells, which is known as an autoimmune disease.