Sequential process of protective immunity against microbial infections in mice.

The sequence of host-defense against microbial invasion consists of nonspecific active substances which are distributed diffusely in the body fluids and exert their functions quickly to eliminate microorganisms, nonspecific humoral elements capable of exerting their functions selectively at the site of microbial invasion, neutrophils, scavenger macrophages, protection at an early stage by IgM antibody or a type of cell-mediated immunity (primitive T cell response) and protection at a late stage by typical immunities. The nature of primitive T cell response distinctive from those of typical immunities are described and its role in the host-defense is discussed.