A person who has been ill with a disease is less likely to get the disease again than someone who never had it. This is because when a person is sick, the body makes infection-fighting antibodies. After the person recovers, these antibodies move around in the person's body and watch for that disease to reappear. If the antibodies detect disease, they quickly signal the body to start making more antibodies to fight the infection. This person is considered to be immune from the disease and may not even know that he or she was exposed to it again.
A vaccine works like the first encounter with disease in that it allows the body to make antibodies that will circulate and watch for the disease to come again. The difference between a vaccine and the first encounter with a disease is that the vaccine causes immunity without causing illness.
View a video clip about how someone becomes immune to a disease, by selecting "What is immunity?" in the "Vaccines and Your Baby" video»
The immune system is composed of two parts: innate and adaptive.
The innate immune system is the fast-acting first line of defense and affords nonspecific immunity through both physical and chemical means. Examples of physical components of innate immunity include our skin and nasal hairs. Chemical components include things like acids in the stomach, enzymes in sweat and saliva, and inflammatory responses that cause heat, redness, swelling and pain locally.
The adaptive immune system takes longer to activate, but responds specifically to an invading pathogen. Antibodies generated by B cells are examples of the adaptive immune response. Immunological memory, in which our bodies remember previous pathogens and activate more rapidly, is also an example of adaptive immunity. Vaccines employ the adaptive immune system to protect us from future encounters with viruses and bacteria.
Reviewed by: Paul A. Offit, MD
Date: February 2013
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