Immunoecological succession in host-parasite communities.

From the unique perspective of the parasite, each potential host exists as an ecosystem in its own right, comprised of biotic and abiotic components. If the parasite's limits of tolerance are not exceeded by the host environment, parasitism may occur. The organs of the host can be viewed as integrated communities comprised of distinct cell populations. Each population of cells occupies a unique ecological niche and contributes to the community in an essential way. The coordinated interactions among cells within each community and among the communities themselves are essential for the host to maintain itself in a homeostatic state. As with free-living systems in equilibrium, introduction of an exotic population into the community disrupts the balance, and ecological succession resumes until a new equilibrium is established. Such is the case when parasites invade the host. The successful invader must not exceed the carrying capacity of the environment nor modify the habitat so drastically that its own survival is compromised. The specific interactions that occur between invading parasites and the populations of cells that comprise the host's immune system can also be viewed in ecological terms. The successful parasite must "compete" with cells in the host community for available niches and avoid "predation" by cells of the host immune system. As is true for interactions among organisms in a free-living ecosystem, the outcome of the host-parasite interaction is not readily predicted from knowledge of the component parts in isolation.