Human neutrophils have outstanding capabilities to sense chemical gradients and follow them to the source
Neutrophils are the white blood cells that protect us against infections. Whenever microbes enter a tissue, say through a small wound, neutrophils spring into action, start moving, and zero in on these microbes. Neutrophils do not have a brain (a neuron is much larger than a neutrophil) and cannot take advantage of any computer. They are only 10 microns in size and yet, they can do all the amazing sensing and movement. The movie above shows neutrophils moving through an artificial maze, towards a source of chemicals similar to those produced by microbes. The chemical is a small peptide, just three amino-acids in size: phenil, methyl, leucine. This peptide is the product of bacteria metabolism the same way as carbon dioxide (CO2) is the by product of human metabolism. The maze between the place where neutrophils are (left compartment) and where the peptide is (right compartment) is not a small challenge. Channels are 5 x 5 microns, smaller than the cells, and so the neutrophils have to squeeze just to fit in. Then there is the orthogonal grid of channels, 50 microns apart. This is approximately 10 million times smaller than the street arrangement in downtown Chicago, but the challenge is the same (if you ever had a hard time finding your way downtown Chicago, you know what I mean). A gradient of the small peptide forms through this network of channels by diffusion. Neutrophils are sensitive enough to pick up the direction of the gradient and go to the source on the shortest route, making little or no mistakes. And, if you watch closely, you will notice that neutrophils are also pretty good at avoiding the traffic jam when many enter the maze in a short time, and picking alternative routes to get to their target.