Parasite dose determines the Th1/Th2 nature of the response to Leishmania major independently of infection route and strain of host or parasite.

Leishmania major causes cutaneous leishmaniasis in mice and man. Infection of mice with relatively low or high numbers of parasites leads respectively to parasite containment, associated with a Th1, cell-mediated response, or progressive disease, associated with a Th2, antibody response in all circumstances studied. These include different parasite strains, different routes of infection, and different hosts previously classified as susceptible, resistant or of intermediate susceptibility. This dose dependency appears to reflect a general rule. We argue that this rule may allow the design of a vaccination strategy that is effective among a genetically diverse population, and that it imposes severe constraints upon proposals for the nature of the "decision criterion" determining whether antigen induces a Th1 or Th2 response.