How does the immune system work?

The immune system is the body's defense network against pathogens. It has two main arms: innate and adaptive immunity. The innate immune system provides immediate, non-specific defense – barriers (skin, mucus), phagocytes, natural killer cells and inflammatory responses. The adaptive immune system is specific: B and T lymphocytes recognize specific antigens, learn (memory cells), and produce targeted antibodies or killer cells. Together they identify invaders (viruses, bacteria) and eliminate them. Innate immunity acts first; if pathogens persist, adaptive immunity ramps up, "remembering" the threat for faster response on re-exposure.