The typhoid bacillus produces a mild diffusible toxine, partly within the intestinal tract, partly within the blood and organs of the body.
This toxine produces proliferation of endothelial cells which acquire for a certain length of time malignant properties.
The new-formed cells are epithelioid in character, have irregular, lightly staining, eccentrically situated nuclei, abundant, sharply defined, acidophilic protoplasm, and are characterized by marked phagocytic properties.
These phagocytic cells are produced most abundantly along the line of absorption from the intestinal tract, both in the lymphatic apparatus and in the blood-vessels.
They are also produced by distribution of the toxine through the general circulation, in greatest numbers where the circulation is slowest.
Finally, they are produced all over the body in the lymphatic spaces and vessels by absorption of the toxine eliminated from the blood-vessels.
The swelling of the intestinal lymphoid tissue of the mesenteric lymph nodes, and of the spleen is due almost entirely to the formation of phagocytic cells.
The necrosis of the intestinal lymphoid tissue is accidental in nature and is caused through occlusion of the veins and capillaries by fibrinous thrombi, which owe their origin to degeneration of phagocytic cells beneath the lining endothelium of the vessels.
Two varieties of focal lesions occur in the liver: one consists of the formation of phagocytic cells in the lymph spaces and vessels around the portal vessels under the action of the toxine absorbed by the lymphatics; the other is due to obstruction of liver capillaries by phagocytic cells derived in small part from the lining endothelium of the liver capillaries, but chiefly by embolism through the portal circulation of cells originating from the endothelium of the blood-vessels of the intestine and spleen. The liver cells lying between the occluded capillaries undergo necrosis and disappear. Later the foci of cells degenerate and fibrin forms between them. Invasion with polymorphonuclear leucocytes is rare.
Many of the phagocytic cells pass through the liver and lungs, and get into the general circulation. A few come from the abdominal lymphatics through the thoracic duct.