A small glandular organ that is situated behind the top of the breastbone, consisting mainly of lymphatic tissue and serving as the site of T cell differentiation. The thymus increases gradually in size and activity until puberty, undergoing involution thereafter.

Excerpted from The American Heritage Dictionary of the English Language, Third Edition Copyright © 1992 by Houghton Mifflin Company. Electronic version licensed from Lernout & Hauspie Speech Products N.V., further reproduction and distribution restricted in accordance with the Copyright Law of the United States. All rights reserved.

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