Your immune system is a complex array of organs, cells and molecules distributed throughout your body. Each part of the system contributes to the growth, development or activation of lymphocytes, sophisticated white blood cells that play a major role in your immune response. White blood cells originate in bone marrow. Some then migrate to your thymus where they develop into specialized types of immune cells.
From bone marrow and thymus, some white blood cells gather
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for example, your immune system stored a few B and T memory cells for chickenpox. The next time you contacted the virus, memory cells multiplied rapidly to stop the infection before it started.
Vaccines also work because of your immune system's ability to "remember." When you're vaccinated, killed or weakened live forms of an infectious organism stimulate an immune response without causing the accompanying illness. Memory cells provide immunity for years or even your lifetime.