Events Occurring Immediately After the Entry of the Microorganism

This chapter discusses the events and responses that follow immediately after the entry of an organism in a host cell. The first activity that could be observed is the growth of the epithelial cells. At the site of entry into the body, microorganisms that are shed directly to the exterior multiply in the epithelial surface producing a spreading infection in the epithelium. Certain microorganisms spread through the body to reach susceptible target organs after free microorganism particles have entered vessels below the skin or intestinal epithelium. The infected host has a variety of defenses that operate without delay, before the immune response comes into action. However, many microbes have strategies for interfering with these defenses. After traversing the epithelial cell layer, a microorganism encounters the basement membrane. After reaching the subepithelial tissues they are exposed to three important host defense systems: the tissue fluids, the lymphatic system leading to the lymph nodes, and phagocytic cells. In addition to being able to resist host defense mechanisms, a pathogenic organism—be it an obligate intracellular, facultative intracellular, or extracellular pathogen—must also overcome the problem of obtaining essential nutrients if it is to be successful.