Lines of defence, the immune response and memory

The human body has two systems of immunity that work together: natural immunity (non-specific, also called innate) and acquired immunity (specific, also called adaptive). Natural immunity is quick and works against any microbe or foreign body, not against one particular kind. It has two lines of defence. The first line is made of barriers that keep pathogens out. The skin has a tough horny layer that is hard to get through. Earwax is made by the ear and kills microbes. Tears contain enzymes that dissolve microbes and so protect the eye. Mucus in the airways is a sticky fluid that traps microbes and foreign bodies breathed in, and the beating cilia of the lining sweep the mucus and what it holds out of the body. Saliva contains substances that kill microbes and enzymes that dissolve them. The stomach lining makes strong hydrochloric acid, which kills microbes that arrive with food. The second line acts if pathogens get past the first, for example through a cut in the skin. It works inside the body and is still non-specific, and it starts with the inflammatory response. When tissue is damaged, certain white blood cells (mast cells and basophils) release large amounts of histamine and similar substances. The blood vessels widen, and the walls of the smallest vessels let more of the blood fluid, the plasma, leak out, so the tissue swells. This also lets chemical messengers and white blood cells, such as neutrophils, monocytes, macrophages and natural killer cells, reach the injury and fight the pathogens. Whether a wound or a swelling needs care is for a doctor to decide.