Gross changes in the size of the secondary lymphoid organs were studied during infection with the nematode parasite Nematospiroides dubius. In the strong responder NIH strain, the wet weight of the mesenteric lymph nodes (MLN) increased rapidly following infection with 400 larvae to peak on day 28 at approximately three times the resting weight. Enlargement of the spleens was also marked but regression to normal size took place when the MLN had achieved maximum size. In contrast in C57BL/10 mice, a slow responder strain, the enlargement of the MLN following infection was relatively slow, and there was no evidence of the regression of the spleen, once maximum enlargement had been achieved.
When adult worms were removed by anthclmintic, the enlarged MLN and spleens returned rapidly to normal size. I lowever, in mice infected with irradiated larvae (25 krad) the MLN stayed enlarged, despite the absence of adult worms but the spleens of these mice returned to normal size fairly rapidly. It was suggested that irradiated worms survive, perhaps as arrested larvae in the intestinal tissue, for a fairly long time, thereby providing a continual stimulus for the MLN.