I've been seeing this all over the place, most recently at Pharyngula, and meaning to play along:

1. Grab the nearest book.
2. Open to page 23.
3. Find the fifth sentence.
4. Post the sentence on your blog along with these instructions.

"This produces lymphocytes, each bearing a distinct receptor, so that the total repertoire of receptors can recognize virtually any antigen."

The antecedent of "this" is recombination of variable receptor gene segments and clonal selection of lymphocytes; the sentence is from Janeway, Travers, Walport and Shlomchik, Immunobiology 5th edition 2001 (Garland ISBN 0-8153-3642-X). If you want a good introduction to immunology, you can't go past this book; it really is first class. Both text and figures present a clear, concise but comprehensive and superbly well organised overview of the field. Highly recommended. You can see for yourself here at the NCBI Bookshelf, although you'll need to come up with searches (base 'em on the chapter headings is the easiest way) because there's no way to browse.