Immune response

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Immune system structures

Phagocytosis

The immune response is how your body recognizes and defends itself against bacteria, viruses, and substances that appear foreign and harmful to the body.

Innate immunity; Humoral immunity; Cellular immunity; Immunity; Inflammatory response; Acquired (adaptive) immunity

The immune system protects the body from potentially harmful substances by recognizing and responding to so-called antigens. Antigens are large molecules (usually proteins) on the surface of cells, viruses, fungi, or bacteria. Non-living substances such as toxins, chemicals, drugs, and foreign particles (such as a splinter) can be antigens. Substances that contain these antigens are recognized and destroyed by the immune system. Even your own body cells have proteins that are antigens. These include a group of antigens called HLA antigens. Your immune system learns to see these antigens as normal and does not usually react against them.

BARRIERS AND INNATE IMMUNITY

Your immune system includes barriers that keep harmful materials from entering your body. These barriers -- part of your innate (with you from birth) immunity -- form the first line of defense in the immune response. Some of these barriers are the skin, stomach acid, mucus (which traps bacteria and small particles), the cough reflex, and enzymes in tears and skin oils. If an antigen gets past the external barriers, it is attacked and destroyed by other parts of the immune system.

BLOOD COMPONENTS

The immune system includes certain types of white blood cells. It also includes chemicals and proteins in the blood, such as complement proteins and interferon. Some of these directly attack foreign substances in the body, and others work together to help the immune system cells.

INFLAMMATION

The inflammatory response (inflammation) occurs when tissues are injured by bacteria, trauma, toxins, heat, or any other cause. Chemicals including histamine, bradykinin, serotonin, and others are released by damaged tissue. These chemicals cause blood vessels to leak fluid into the tissues, causing swelling. This helps isolate the foreign substance from further contact with body tissues.