Vitellogenin is a novel player in defense reactions.

The significance of the presence of vitellogenin (Vg), a female-specific protein, in male and juvenile fish is uncertain. Here we show that the Vg purified from the rosy barb Puntius conchonius possesses both antibacterial and hemagglutinating activities in vitro, and the male fish challenged with Escherichia coli synthesize Vg. These suggest that in addition to being involved in yolk protein formation, Vg is also involved in defense reactions, a novel function assigned to Vg.