Immune Evasion by bacteria
How do bacteria circumvent the immune system?
Obviously, if the immune system successfully recognises, consumes and destroys all of the organisms that are present in any given infection, then that organism has failed to establish itself in the body, and thus cannot cause disease.
However, the immune system does not always succeed in its task. Often the reason for this failure is that the invading organism has evolved a strategy for evading or suppressing the hosts immune response to that organism.
Many different strategies have evolved in different organisms; below is presented a list of some of those strategies.
Strategies directed against acquired immunity
- Molecular mimicry. Organisms that employ this strategy genetically resemble parts of the hosts own body. The body is less likely to allow the existence of immune antibodies that act against the body itself (i.e. autoimmunity), since that can be highly dangerous to the body. By mimicking the genetic/chemical makeup of the body, organisms can exist without an immune response being activated against them. The bacterium Bacteroides, for example, can avoid evoking an immune response in mice. There are several bacteria that mimic parts of the human body, but these usually give rise to an immune response. Since the resultant anibodies act against both the foreign organism and the bodies own tissues, this has the effect of making the body mount an immune response against itself. Mycobacterium tuberculosis, Mycobacterium leprae, Treponema pallidum, Mycoplasma pneumoniae are all organisms that cause autoimmunity in humans. Mycobacterium paratuberculosis has been demonstrated to use this strategy in animals.
- Suppression of antibodies. In this strategy, the invading organism employs the strategy: the best form of defense as attack. The organisms in this category target those cells of the immune system that specifically react against them. By disabling those cells, the organism prevents the body from mounting an immune response against the invader. Two notable bacteria that suppress the bodies reaction against them are Mycobacterium leprae, the cause of the disease leprosy, and Mycobacterium tuberculosis, the cause of the disease tuberculosis. In both cases, the Interleukin-2 response to the bacteria is reduced. In lepromatous leprosy, it has been shown that suppressor T cells, taken from the skin lesions caused by the disease, inhibit the responses of other T cells to Mycobacterium leprae antigens.
- Hiding inside cells. Many bacteria avoid an immune response by hiding inside the cells of the immune system. By doing so, they do not present antigens that will evoke an immune response. They multiply inside these cells, and then further invade the body when they are greater in number. Among the bacteria that use this strategy are Brucella, Listeria, Mycobacterium leprae and Mycobacterium tuberculosis, all of whom infect macrophages, the cells that are normally responsible for destroying invading bacteria. Mycobacterium leprae can also invade and inhabit cells that are not a part of the immune system, e.g. skin cells.
- By releasing antigen into the bloodstream. Normally, the antigen of an invading organism that is recognised by the body exists on the cell wall of that organism. Some organisms, however, release these antigens from their cell walls to float freely in the bloodstream, where eventually they will meet their antibody, and will bind to them, thus rendering those antibodies ineffective. To provide an analogy, if the immune system were to recognise only the "face" of an organism, then organisms that use this strategy not only shed their "faces", but also produce as many "faces" as they can, which act as decoys and confuse the immune system. This leaves the rest of the organism freedom to move about unhindered.
Strategies directed against Phagocytes
As you remember, the Phagocytes, i.e. the Macrophages and Microphages, are the cells responsible for consuming and destroying invading bacteria. Many microbial strategies for survival involve action against the Phagocytes. Strategies include
- Inhibiting Chemotaxis. Chemotaxis is the chemical process by which the Phagocytes are led to the site of infection, so that they can begin their task. Some bacteria, such as Staphylococcus Aureus, produce toxins which inhibit the movement of Phagocytes, which hinders them in their journey.
- Inhibiting Phagocytosis. In order to Phagocytose (i.e. swallow and consume) bacteria, phagocytes must first grip onto the invader, and engulf them. Some bacteria evade Phagocytosis by not presenting anything for the phagocytes to grip onto.
- Killing the Phagocyte. Some bacteria are capable of releasing toxins that are lethal to Phagocytes. So instead of the invading bacteria being destroyed, the defending phagocytes are themselves destroyed. Among the bacteria that are capable of this strategy are Mycobacterium tuberculosis, Streptococcus pyogenes, some Staphylococci, and Bacillus Anthracis (the bacterium that causes the disease Anthrax).
- Colonising the Phagocyte. This highly successful strategy involves the invading bacteria allowing themselves to be Phagocytosed, but resisting being killed once they are inside the Phagocyte. Many types of bacteria use Macrophages as sites of sanctuary, where they can multiply without interference from other cells of the immune system. As mentioned above, bacteria that use this strategy include Mycobacterium leprae and Mycobacterium tuberculosis.