O agglutinin

[ō]
n.
  1. An agglutinin that reacts with, and is formed as the result of stimulation by, the relatively thermostable antigens present in the cell bodies of microorganisms.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.