These isotype-defined monoclonal antibodies, derived from a hybridoma producing IgM isotype immunoglobulins through in vitro immunization, had neutralizing in vivo infection properties when passively transferred to BALB/c mice experimentally challenged with lethal doses of Plasmodium berghei and Plasmodium yoelii, in a double-challenge administered by two different routes. The gene discussed is CD40LG; the disease is infection.