The immune system is an intricate network of specialized tissues, organs, cells, and chemicals. The lymph nodes, spleen, bone marrow, thymus gland, and tonsils all play a role, as do lymphocytes (specialized white blood cells), antibodies, and interferon.
Two types of immunity protect the body: innate and adaptive. Innate immunity is present at birth and provides the first barrier against microorganisms. The skin, mucus secretions, and the acidity of the stomach are examples of innate immunity that act as barriers to keep unwanted germs away from more vulnerable tissues.
Adaptive immunity is the second barrier to infection. It is acquired later in life, such as after an immunization or successfully fighting off an infection. The adaptive immune system retains a memory of all the invaders it has faced. This is why people usually get the measles only once, although they may be repeatedly exposed to the disease. Unfortunately some bugs--such as the viruses that cause the common cold--"disguise" themselves and must be fought off time and again by the immune system.