29.2.6 Immunity to intracellular bacteria

Some intracellular bacteria like pathogenic or facultative are able to multiply within the phagocytes, so their elimination from the patients requires modified strategies.

29.2.7 Innate immunity to intracellular bacteria

Phagocytes and natural killer cells provide innate immunity to the intracellular bacteria. However some bacteria survive and multiply easily in the phagocytes, the phagocytes need to be stimulated by the secretions of these bacteria in order to clear the infection. The secretions from these bacteria are recognized by TLRs and cytoplasmic proteins of the NOD-like receptor (NLR) family so that they stimulate the phagocytes to degrade the invading bacteria. In addition to the intracellular bacteria, activated natural killer cells produce IFN-γ, which consecutively stimulates macrophages and cytokines. Although innate immunity provides protection from most of the bacteria but some intracellular bacteria like Listeria monocytogenes need cell mediated immunity in order to be eliminated from the body.

29.2.8 Adaptive immunity to intracellular bacteria

T cell-mediated immunity plays a significant role in providing protection against intracellular bacteria. CD4+ T-cells and CD8+ cytotoxic T lymphocytes are the two major forms of cell mediated immunity that participate in phagocytosis or killing of infected cells, respectively. Both the, CD4+ T-cells and CD8+ cytotoxic T lymphocytes work together to provide protection against the intracellular bacteria. Granulomatous inflammation acts as a marker for most of the infections due to intracellular bacteria, which occurs because of T-cell and macrophage stimulation. Macrophage stimulation that occurs as an antigenic response towards intracellular microbes is sometimes able to cause tissue damage. The response shown by different patients towards the intracellular microbes decides the development of the disease and its consequence. One neat example of such type of response is shown by leprosy patients. Leprosy is a disorder caused by Mycobacterium leprae and it exists in two forms, the lepromatous and tuberculoid form. Lepromatous form is characterized by feeble cell-mediated immune response and high specific antibody titer while the tuberculoid form shows low specific antibody titer but very strong cell-mediated immune response. Although the reasons attributed to this type of response are still speculated and not yet verified, one of the factors that are given significance is regarding varied pattern of cytokine production and T-cell differentiation in patients.

29.2.9 Dodging of immune system by intracellular bacteria

Intracellular bacteria tend to dodge the immune system in many ways comprising evading into the cytosol or preventing phagolysosome fusion and by overpowering the reactive oxygen species by their microbicidal activity. These bacteria have the potential to cause chronic infections because they can survive the phagocyte mediated elimination and thrive for years in the body and may show reversion of the disease.