Pathogens, Host-Cell Invasion and Disease

Invading pathogens can co-opt even the cells of the immune system. New anti-infective drugs may arise from an understanding of this chemical warfare

Erich Gulbins, Florian Lang

Pathogens and Their Hosts

Organisms have always been attacked by pathogens; the species that have survived, including us, are those that have evolved means to fend off their attacks. Likewise, pathogens have developed means to overcome these protective measures, forcing the evolution of even more powerful host defenses. Over millions of years, the arms race between pathogenic organisms and animals eventually created the mammalian immune system, which includes a wide variety of highly specialized compounds (toxic proteins that attack foreign cells marked with antibodies) as well as immune cells—the large macrophages of the blood and lymph, the natural killer cells, T lymphocytes and antibody-producing B lymphocytes. Immune cells patrol the bloodstream, various organ tissues, the surfaces of the intestines and the respiratory and urinary tracts.

No matter where a pathogen comes from—via the respiratory or the digestive tract, an injury or an insect bite—it faces an army of immune cells that are specialized to devour invading pathogens, shower them with toxic chemicals, punch holes into their cell membranes or bombard them with antibodies to mark them for destruction. In fact, this defense system is so effective that most people spend most of their lives free of infectious disease, despite daily exposure to countless viruses, bacteria and infectious eukaryotic parasites (microorganisms with nucleated cells, such as Plasmodium, the protozoan that causes malaria).

But pathogens, having the advantage of larger numbers, can overwhelm the host's immune system and often adapt more nimbly. Mutation and selection have provided them with mechanisms for circumventing or countering the immune system's attacks. Many pathogens, notably viruses but also many bacteria and eukaryotic microorganisms, invade host cells as a first step so that they are hidden from the immune system as they multiply. However, invasion is not a one-sided process; it often requires the active participation and cooperation of the host. The properties of both host and pathogen determine whether the pathogen can successfully establish an infection or whether it is killed by the host.