Within about four hours of the mosquito bite, many of the lymph-node parasites appeared degraded. They were also seen interacting with key mammalian immune cells, suggesting that the immune cells were destroying them.
A small number of the parasites in the lymph nodes, however, escaped degradation and began to develop into forms usually found only in the liver. Up to now, researchers believed that, although both blood and lymphatic vessels take up Plasmodium parasites, they all end up in the liver, Ménard said. "Nobody had proposed that they actually might stop" in the lymph nodes and develop there, he observed.
By 52 hours after the mosquito bites, no parasites remained in the lymph nodes, which suggests that they can't develop completely there, Ménard said. Only fully developed parasites can infect red blood cells and cause malaria, so the lymph-node parasites probably don't contribute to the appearance of malaria symptoms, he added. But even partially developed or destroyed parasites could significantly affect how the immune system responds to infection, he noted.
Another unexpected finding adds even more complexity to the mammalian immune response to the malaria parasite. An hour after a mouse was bitten, nearly half of the parasites remained in the animal's skin, and some were detected there even after seven hours. "That's really surprising," Ménard said.
Although he cautions that those numbers may be specific to mice and the species of Plasmodium the scientists used, it's likely that at least some parasites remain in the skin of any mammal bitten by a malarial mosquito until immune cells come along to sweep them out, Ménard said. This second influx of parasites could prompt a somewhat different immune response in the host, and those parasites might have different fates. Parasites developing in the lymph nodes could have two opposite effects
Source:Howard Hughes Medical Institute