Although some studies have indicated that the tonsil is the site
of primary infection, at least through experimental oral
challenge (107), other reports have not confirmed this
conclusion (51). The organism is generally believed to cross
from the lumen of the small intestine into the lymphoid
system via the M cell (97). The bacterium is then taken up by
epithelioid macrophages which, once activated, elicit T cell
activation and clonal expansion (125). Two T helper cell
subpopulations (TH1 and TH2) activate different host
immune responses. Mycobacterium paratuberculosis infection
appears to follow patterns observed with infections by
M. leprae, M. bovis or M. tuberculosis. These patterns entail an
initial TH1 response (referred to as ‘tuberculoid’) that is
characterised by a tissue infiltrate distinguished primarily by
lymphocytes with few if any detectable organisms (101). The
TH1 response is also characterised by production of the
cytokine gamma interferon (IFNg), one of the earliest
detectable reactions to M. paratuberculosis infection, in
addition to interleukin 2 (IL-2) and tumour necrosis factor
alpha (TNF-a). These cytokines are believed to orchestrate
the cell-mediated immune functions necessary to contain
Although some studies have indicated that the tonsil is the siteof primary infection, at least through experimental oralchallenge (107), other reports have not confirmed thisconclusion (51). The organism is generally believed to crossfrom the lumen of the small intestine into the lymphoidsystem via the M cell (97). The bacterium is then taken up byepithelioid macrophages which, once activated, elicit T cellactivation and clonal expansion (125). Two T helper cellsubpopulations (TH1 and TH2) activate different hostimmune responses. Mycobacterium paratuberculosis infectionappears to follow patterns observed with infections byM. leprae, M. bovis or M. tuberculosis. These patterns entail aninitial TH1 response (referred to as ‘tuberculoid’) that ischaracterised by a tissue infiltrate distinguished primarily bylymphocytes with few if any detectable organisms (101). TheTH1 response is also characterised by production of thecytokine gamma interferon (IFNg), one of the earliestdetectable reactions to M. paratuberculosis infection, inaddition to interleukin 2 (IL-2) and tumour necrosis factoralpha (TNF-a). These cytokines are believed to orchestratethe cell-mediated immune functions necessary to contain
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