Microorganisms and their interaction with the immune system.

Microorganisms interact with the immune system in multiple ways. In an interaction between a microorganism and its host, the defense of the host does not go unchallenged. Microorganisms have for decades been suspected of possessing the capabilities of hiding from and escaping the consequences of immune surveillance. Escape mechanisms like antigenic variation, latency, and genomic integration can best be described as passive mechanisms for avoiding interaction with the host immune system, to differentiate them from the more engaging and host-directed active mechanisms of interaction. Studies of the mechanism of direct entry of viruses (HIV, measles, and enteroviruses), bacteria (streptococci and staphylococci), and parasites (Leishmania and plasmodium) into immune cells like CD4+ T cells or macrophages, as reported very recently, indicate an even more aggressive mode of interaction. This aggressive mechanism of interaction with the components of the host immune system allows the microbe not only to block the normal function of immune components on the surface of immune cells from functioning, but also to obliterate a vital immune function, cellular immunity, causing immunosuppression, e.g. the depletion of CD4+ T cells due to the entry and replication of HIV. Collectively, microorganisms have evolved various mechanisms by which they can actively block almost any cellular, humoral, or systemic immune response. One general feature of the proteins that assist microorganism to immunomodulate and actively evade host defense is their structural and therefore functional similarity to the host proteins, which they effectively mimic. Understanding the different mechanisms by which microorganisms interact with the immune system can impact the design of live vaccines as well as the development of novel therapeutic immunomodulators that can provide medicine with powerful new tools to manage immune disorders, allograft rejection, remote multiple organ failure resulting from trauma, autoimmune diseases, etc.