Activated germinal centre B cells undergo directed migration.

Affinity maturation, the fundamental basis for adaptive immunity, is accomplished through somatic hypermutation of B-cell receptors followed by expansion of rare mutants with higher affinity for the immunising antigen. This process occurs over a period of weeks in unique micro-anatomic sites known as germinal centres. Two-photon microscopy has recently made it possible to track individual cells moving within germinal centres in living animals. Here we apply statistical approaches to test the hypothesis that B-cell motion is random. Our results show that activated B cells move in a directed manner that sharply contrasts with the behaviour of naïve B cells.