'Then they must have an efficient immune response!' (Boman)." Their work asked how insects can survive infections without having a system of B cells and T cells, at the time thought to be essential for immune defence.[3] They examined the immune system of fruit flies and could show that flies that were once infected with a non-lethal dose of the bacterium Pseudomonas survived a second higher dose while flies that received the same high dose for the first time died.