The immune system defends the body against disease by identifying and destroying microorganisms and substances that could cause harm. The immune system can recognize millions of different potential invaders and can maintain the ability to recognize them over decades.
Structure
The immune system is made up of immune cells, known as leukocytes, and the tissues and organs that support immune function. Bone marrow produces the immune cells that fight infections. These immune cells are stored in the lymph nodes and in the lymphoid organs, the spleen and thymus gland, explains the Nemours Foundation.
Cell Types
Leukocytes come in a few different varieties. The two types of leukocytes are phagocytes and lymphocytes. Phagocytes destroy invaders, or damaged cells, by engulfing and destroying them and are further divided into different cell types including neutrophils, which destroy bacteria, and macrophages, which devour dead cells and cell debris. Lymphocytes are divided into B cells, which produce antibodies, and T cells, which destroy anything tagged with those antibodies or call in phagocytes to devour them, according to the National Institute of Allergy and Infectious Disease.
Purpose and Action
The immune system has many functions, all with the express purpose of protecting the body from foreign invaders. The immune system contains cells that can recognize and destroy foreign antigens and avoid attacking substances made by the body itself. Regulatory cells within the T cell population work to control the immune response and prevent it from damaging other tissues and cells. Different immune cells work together to mobilize the response against invaders and memory cells operate to maintain immunity so that if the immune system later encounters the same antigen the immune response will be rapid enough to prevent infection or disease.
Types of Immunity
Innate immunity is the natural immunity everyone is born with that prevents people from getting sick whenever encountering anything that is different, such as diseases from cats and dogs. Innate immunity operates because the cells of the body don't respond to these antigens and so the immune system can simply clear them out of the body as debris. Acquired immunity occurs when the immune system encounters a disease-causing antigen and destroys it, but maintains the memory of that type of antigen for future protection. Acquired immunity can come from catching diseases or from vaccination. Passive immunity is short-term immunity granted by some outside source, such as the breast milk that an infant consumes, which offers protection against some diseases.
Disorders
Many different disorders can strike the immune system, explains the Nemours Foundation. Immune system cancers such as lymphoma and leukemia destroy the immune system's ability to fight disease as the cancer cells outgrow and overcome the healthy ones. Allergic disorders occur when the immune system overreacts to allergens or treats something harmless, such as compounds in foods, as an allergen. Autoimmune disorders occur when the immune system attacks tissues of the body. Examples of autoimmune diseases include arthritis and lupus. Immunodeficiencies happen when certain immune cells don't work properly or the bone marrow fails to produce them. These can include biological or genetic problems such as IgA deficiency disease or immunodeficiency caused by infections such as HIV.
