1. Facultad de Ciencias, Universidad Autónoma de Baja California, Km 108, Carretera Transpeninsular, Ensenada, Baja California, México
2. Current address: Division of Immunology and Cell Biology, The John Curtin School of Medical Research, The Australian National University, P.O. Box 334, Canberra City ACT 2601, Australia (e-mail: ).