The immune system is a large network of organs, white blood cells and proteins that work together to defend the body from germs that cause diseases. It also helps the body heal from infections and injuries.
The two main parts of the immune system are the innate and adaptive immune responses. The innate immune response occurs when you come into contact with a germ that makes you sick. It is a fast-acting but not permanent form of immunity. The adaptive immune response is the body’s slower, more permanent form of immunity.
Adaptive immunity involves two kinds of lymphocytes: B cells and T cells. Lymphocytes are cells that develop and mature in the bone marrow, or in special deposits in the spleen and lymph nodes. They then travel to the thymus gland to further mature, or to the periphery of the body where they develop into T cells and B cells.
B cells recognize antigens on the surface of germs, or on other cells in the body. They bind to these antigens and send out signals that prompt the rest of the immune system to attack the pathogen. T cells then destroy the pathogen or mark it for destruction by other immune system cells.
Some T helper cells become memory T cells, which remember the specific antigens that a particular germ has and will react very quickly to those germs in the future. This memory is what gives you natural immunity to a disease.