Natural (innate) immunity

• A group of defences that are fast and effective, aimed at any microbe or foreign body, not at a specific microbe or antigen.
• They form two lines of defence.

First line – physical and chemical barriers

They prevent pathogens from entering the body:

• Skin – tough horny outer layer difficult to penetrate; sweat is salty and kills most microbes.
• Cerumen (ear wax) – secreted in the ear; kills microbes and protects the ear.
• Tears – contain enzymes that lyse (dissolve) microbes, protecting the eye.
• Mucus in the respiratory tract – sticky fluid lining the airways that traps microbes and foreign bodies in the inhaled air; the beating cilia sweep the mucus with its trapped microbes out of the tract.
• Saliva – contains microbe-killing substances and enzymes that digest microbes.
• Gastric juice – the stomach lining secretes strong hydrochloric acid that kills microbes swallowed with food.

Second line – internal non-specific defences

• Acts when pathogens have broken through the first line, for example through a cut in the skin.
• It uses a series of non-specific mechanisms to surround the invaders and prevent spread, beginning with the inflammatory response:

1. Damaged tissue and microbes cause certain white blood cells – mast cells and basophils – to release histamine (and other inflammation-inducing substances).
2. Histamine makes the local blood vessels dilate greatly and makes arterioles and capillaries more permeable.
3. Blood fluid (plasma) leaks from the vessels into the tissue, causing swelling; the extra blood flow causes redness, heat and pain.
4. Increased permeability allows chemicals such as interferons to reach the site and attack or dissolve pathogens, and lets white blood cells – neutrophils, monocytes/macrophages and natural killer cells – leave the capillaries and fight the pathogens.

Benefit of having several kinds of white blood cells present: each does a different task – neutrophils and macrophages engulf and digest bacteria, NK cells destroy infected cells, others secrete chemicals – so together they overwhelm the infection faster than any single type could.