Before vaccines were invented, the only way to create immunity in the body was to suffer through a bout of the disease in question. Once endured, providing the patient survived, his or her immune system was more likely to be able to fight off any future infections before they took hold. B-cells in the bloodstream, responsible for fighting off the disease, retain memory of the illness. If it returned, the immune system could launch a quick attack.
Vaccines produce the same effect without making the patient suffer through the disease. By introducing a disease into the bloodstream, B-cells are stimulated into action, creating antibodies and a memory record of the pathogen, resulting in immunity. This preventative treatment is safe because the viruses or bacteria used have been severely compromised in a laboratory by one of several strategies.
In the case of measles, mumps, and chickenpox, the virus's ability to reproduce rapidly is weakened. Normally, these viruses cause illness by reproducing themselves thousands of times in the body. One used in a vaccine may reproduce itself 20 times or less, but this is still enough to initiate B-cell reaction and cell memory. One or two injections of a live, weakened virus will typically make a person immune for life. Unfortunately, this type is not safe for people with compromised immune systems, such as those suffering with cancer or AIDS.
Another strategy is to deactivate a virus by killing it with a chemical. The virus will not be able to reproduce at all, yet its presence in the body still generates a response by B-cells, producing antibodies and a memory record. As safe as this method is for people with weakened immune systems, however, its drawback is that it takes several treatments to achieve long-lasting immunity. Vaccines of this type include polio, influenza, hepatitis A, and rabies.
For hepatitis B, only a part of the virus is used: a protein taken from the surface of the virus. In this case, B-cells respond to the protein, immunizing the body against the entire virus. This is safe for people with weakened immune systems, but it requires three doses for lifetime immunity.
Several bacterial diseases have vaccines based on using part of the bacteria as well. These bacteria make harmful proteins, called toxins, that can be inactivated with a chemical. Once the toxin is inactivated, the bacteria is harmless, and therefore so is the vaccine. Two examples of this type are diphtheria and tetanus.
Many people get the flu shot every year, which contains dead influenza viruses, while the nasal-spray flu vaccine contains live, weakened viruses. The nasal spray is normally given to healthy individuals between age 5 and 49. The flu shot can be administered to anyone 6 months or older, regardless of health. Each shot contains 3 different flu viruses, prevalent that year.
People should always check with a healthcare provider for which vaccinations that are best-suited to their personal health profile.