The immune system is a network of cells, tissues, and organs that defends the body against infection. Its first line is physical: skin, mucus, and stomach acid keep most invaders out. Anything that gets past these barriers meets the innate response, in which white blood cells such as macrophages engulf bacteria and trigger inflammation to bring more defenders to the site. If the threat persists, the adaptive immune system takes over. B cells produce antibodies that lock onto specific molecules on the surface of a pathogen, while T cells destroy infected cells directly or coordinate the wider response.
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