Basics of the Immune System
The human body is constantly bombarded by millions of viruses, bacteria, and other disease-causing microorganisms, or pathogens. Fortunately, most of these are thwarted by the body's own protective physical and chemical barriers, such as the skin, saliva, tears, mucus, and stomach acid.
The millions of bacteria that live on the skin and the body's mucous membranes also help protect against certain invaders. When a pathogen does manage to evade these defenses and enter the body, it is attacked almost immediately by one or more components of the immune system.
The immune system uses extremely sensitive chemical sensors to recognize a foreign organism or tissue, especially one that can cause disease. Sometimes it overreacts to a harmless substance, such as pollen or a certain food or medication; this can set the stage for an allergic reaction. In other cases, the immune system mistakenly attacks normal body tissue as if it were foreign, resulting in an autoimmune disease such as lupus or rheumatoid arthritis. Most of the time, however, the immune system holds fast as our first line of defense against a host of potentially deadly diseases.
Most lymph nodes are clustered in the neck, armpits, abdomen, and groin. Fluid that drains from body tissues into the lymphatic system filters through at least one lymph node, where layers of tightly packed white blood cells attack and kill any harmful organisms. Blood vessels transport white blood cells, antibodies, and other protective substances produced by the immune system. The lymphatic system also returns body fluid to the bloodstream after it has been filtered through lymph nodes.
Disease-causing organisms vary from tiny viruses to parasites such as the tapeworm, which can grow 20 feet long. Regardless of the size or species of the invading organism, a healthy immune system will mount a vigorous defense against it. The exact nature of that defense varies, however, according to the type and number of invading organisms.