The response of the murine lymphoid system to a chronic infection with Trypanosoma congolense. II. The lymph nodes, thymus and liver

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Abstract
The cellular responses in the lymph nodes, thymus and liver of mice infected with Trypanosoma congolense were examined and considered in relation to changes in the level of Ig in the serum. A significant proliferative response occurred in the lymph nodes only after 3 weeks of infection and involved mainly the B‐dependent folliçular areas in which there was marked germinal centre formation. This was associated with the appearance in the medullary cords of large numbers of plasma cells. Only a minor increase in the number of macrophages was observed in the lymph node sinuses and there was no obvious lymphoid depletion. A partial depletion of the cortical areas of the thymus in the later stages of the infection was the only remarkable feature in that organ. An abrupt but transient increase in the levels of the 4 classes of Ig which were monitored was observed about the time of the first peak of parasitaemia and was considered to result from the proliferative response in the spleen. A second and more pronounced increase occurred later coinciding with the onset of proliferative activity in the lymph nodes. There was marked cellular infiltration into the liver; this commenced about the time of the first peak of parasitaemia when it involved mainly lymphocytes and plasma cells and became progressively more severe involving a mixture of cell types including lymphocytes, plasma cells and erythropoietic and granulopoietic cells. In addition, there was an increase in the number of Kupffer cells lining hepatic sinusoids. Throughout most of the infection, focal hepatic necrosis and increased mitotic activity of hepatocytes were prominent features. The differences in timing and character of the cellular responses between the different lymphoid organs were considered to be due in part to the restriction of T. congolense to the intravascular spaces.