The malaria parasite hides inside liver cells. Immune cells have to find and destroy it within two days, or the disease will develop. Researchers used to think the cells sense a chemical trail from the parasite and head straight for it. But scientists tracked cell movement in the livers of mice and saw something different.
Scientists turned the vessel network into a graph, a scheme of points and lines. Then they compared how cells would move during a random search with how they actually moved. It turned out that the cells aren't attracted to the parasite. But random wandering is enough: a single cell finds the infected spot in 12 hours on average. Over two days, the probability of success is almost 100 percent.
So the vascular maze doesn't get in the way; it helps. Thanks to its structure, random search works faster than it would in solid tissue. That means the body doesn't need complex signals for defense: it's enough to simply let the cells wander through the vessels.
The fractal dimension of the sinusoid graph turned out to be about 1.7–2.0, less than the three dimensions of the liver. This means that T cells search for a target in a space whose properties are closer to a plane than to a volume, and therefore random search works faster.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.