Your Immune System

FROM WebMD...

The Immune System is a network of tissues, cells, and organs first tries to keep out germs like bacteria, viruses, fungi, and parasites and then deals with them if they manage to get in. If it senses something in your body that could be bad for you, it triggers the release of special cells. These travel to where the trouble is, attack the intruder, and help get rid of it.

Your body has to be able to stop invaders that come from a lot different places. Germs can come from contact -- touching skin, having sex, and breathing in drops from someone else's sneeze or cough, for example. They can travel through blood that comes from a shared needle or an insect bite. You can also get germs from contaminated food or water.

Your skin is the most obvious one. It blocks invaders from getting into your body in the first place. Other blockers are the clear layer over the front of your eye (cornea) and specialized tissue that lines your lungs, bladder, and digestive system. A cut, sore, or burn can make an opening in any of these for a germ to get in and infect you.

Sweat on your skin, tears in your eyes, and mucus in your nasal passages, digestive system, and a woman's vagina can stop invaders from getting in, too. These liquids your body makes not only push away dirt and germs but also have enzymes that can kill bacteria.

A network of fine tubes throughout your body collects fluid called lymph from tissues. Part of its job is to pick up dead cells and germs. Waste is filtered out at small bean-shaped lymph nodes, and the liquid goes back into your bloodstream. An infection can make the nodes swell. You may have felt them in your neck when you had a sore throat or cough.

These are markers that your immune system can recognize. Some, called human leukocyte antigens (HLA), tag your cells so your body can ID itself. Others could be part of a foreign cell or germ, or they may be a substance like food or pollen.

When you're born, before your body comes across any unfamiliar antigens, it can defend itself from infection. This innate immunity comes from those barrier body parts as well as some specialized cells. Over time, your immune system "learns" other ways to protect you. Acquired immunity comes from antibodies you get from your mother in the womb or that you make in response to antigens that aren't yours -- like from a cold virus or a vaccine.

The soft, fatty stuff that lives inside your bones is where your body makes blood cells, including the various white blood cells that fight off germs.

They're part of your innate immunity, and they work by eating invaders. Neutrophils, the most common type of white blood cell, are among the first responders called to a trouble spot. They digest bad cells and can trap bacteria and stop it from spreading. Macrophages grow from white blood cells called monocytes, but they work in tissues, not your blood. Eosinophils mainly attach to parasites that are too big to ingest in order to kill them.

Another part of your innate immunity is this type of white blood cell. They recognize and latch onto abnormal cells like cancer, then damage and kill them. They're key players when you first get infected by a virus.