Immune system: defenses

Every day your body meets pathogens, microbes that can cause disease: bacteria, viruses, fungi, and parasites. Yet you stay healthy most of the time because of the immune system, a team of cells, tissues, and organs that defends you. Its first defenses work against almost any invader, which is why they are called nonspecific.

The first line of defense keeps pathogens out. Your skin is a waterproof barrier that most germs cannot cross. Mucus in your nose and airways traps germs, and cilia sweep it away. Tears and saliva contain an enzyme that damages bacteria, and stomach acid kills many germs you swallow. Harmless bacteria that live on your skin and in your gut also crowd out harmful ones.

If a pathogen gets through, the second line begins. Phagocytes, a type of white blood cell, engulf and digest invaders. They travel in the blood and squeeze out through capillary walls to reach an infection. When tissue is damaged or infected, cells release chemical signals that start inflammation. Blood vessels near the injury widen and become leaky, which brings more blood and white blood cells to the area. This causes the four classic signs of inflammation: redness, warmth, swelling, and pain. Natural killer cells, another kind of white blood cell, destroy body cells that have been infected by viruses.

Fever is another defense. During an infection, the body temporarily raises its temperature set point. Many germs grow more slowly in warmth, and some immune cells work faster. A mild fever is usually a sign that your body is fighting an infection, not an illness by itself, though a very high fever or one that lasts is a reason to see a doctor.

Not all germs are alike, and the right treatment depends on which kind you have. Antibiotics kill bacteria but have no effect on viruses, so they do not help against colds or the flu. Using antibiotics when they are not needed also helps bacteria become resistant to them. And most microbes are harmless or helpful; only a few cause disease.

Common mistakes: thinking the immune system is only white blood cells, and thinking that the symptoms of illness, such as fever or swelling, are caused only by the germ. Often they are the immune response at work. Next lesson: the targeted response that remembers pathogens.