Immune response

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	Phagocytosis

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

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 antigens. Antigens are molecules (usually proteins) on the surface of cells, viruses, fungi, or bacteria. Nonliving substances such as toxins, chemicals, drugs, and foreign particles (such as a splinter) can also be antigens. The immune system recognizes and destroys substances that contain these antigens.

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Even your own body's 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 usually does not react against them.

INNATE IMMUNITY

Innate, or nonspecific, immunity is the defense system with which you were born. It protects you against all antigens. Innate immunity involves barriers that keep harmful materials from entering your body. These barriers form the first line of defense in the immune response. Examples of innate immunity include:

• Cough reflex
• Enzymes in tears and skin oils
• Mucus, which traps bacteria and small particles
• Skin
• Stomach acid

Innate immunity also comes in a protein chemical form, called innate humoral immunity. Examples include the body's complement system and substances called interferon and interleukin-1 (which causes fever).

If an antigen gets past these barriers, it is attacked and destroyed by other parts of the immune system.

ACQUIRED IMMUNITY

Acquired immunity is immunity that develops with exposure to various antigens. Your immune system builds a defense that is specific to that antigen.

PASSIVE IMMUNITY

Passive immunity is due to antibodies that are produced in a body other than your own. Infants have passive immunity because they are born with antibodies that are transferred through the placenta from their mother. These antibodies disappear between 6 and 12 months of age.