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What is clonal expansion?

Anthony L. Komaroff, MD
Internal Medicine
The explosive increase in the number of lymphocytes, both B cells and T cells, from just a few to millions in the presence of an infection was discovered in the 1950s. The process, called clonal expansion, is what gives the adaptive immune system its extraordinary might and specificity. You can tell that clonal expansion is occurring when you feel tender bumps (swollen lymph nodes) in your neck or other areas.

When lymphocytes multiply during clonal expansion, some of them are destined to live on as memory T and B cells. These clones are a subset of the expanded number of T and B cells that develop from your first exposure to a germ, and they protect you against subsequent attacks by the same germ.

Because of this new population of memory cells, the responses to subsequent attacks are faster and greater than the first. This explains why once you've had an infectious illness, you don't get sick when you're exposed to it the next time around.

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