How does the body protect itself from the majority of pathogens it encounters?

In general, your body fights disease by keeping things out of your body that are foreign. Your primary defense against pathogenic germs are physical barriers like your skin. You also produce pathogen-destroying chemicals, like lysozyme, found on parts of your body without skin, including your tears and mucus membranes. In response to infection, your immune system springs into action. White blood cells, antibodies, and other mechanisms go to work to rid your body of the foreign invader. The immune system has a vital role: It protects your body from harmful substances, germs and cell changes that could make you ill. It is made up of various organs, cells and proteins.

Once infected cells have sensed an invading pathogen, they secrete molecules called cytokines and chemokines. These cells are then primed to resist an infection with the invading virus. Cytokines such as interferons activate anti-viral genes in the infected and neighbouring cells.

Microorganisms capable of causing disease—or pathogens—usually enter our bodies through the eyes, mouth, nose, or urogenital openings, or through wounds or bites that breach the skin barrier. Organisms can spread, or be transmitted, by several routes.

The second line of defense is nonspecific resistance that destroys invaders in a generalized way without targeting specific individuals: Phagocytic cells ingest and destroy all microbes that pass into body tissues. For example macrophages are cells derived from monocytes (a type of white blood cell).

If pathogens do manage to enter the body, the body's second line of defense attacks them. The second line of defense includes inflammation, phagocytosis, and fever.

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