LeukoChase: Bloodstream Racer

A nod to Deathchase (Micromega, 1983) on the ZX Spectrum. Instead of chasing riders through a forest on a motorbike, you chase pathogens through the blood as a white blood cell, in three stretches: an artery, the liver, then the kidneys and the veins back towards the heart.

The science

• You are a macrophage, a white blood cell that eats microbes by phagocytosis. A swallowed microbe ends up in a phagosome, which fuses with lysosomes full of digestive enzymes such as proteases: the shots you fire. Strictly, the white cells patrolling the blood are monocytes, which become macrophages once they leave the vessel for the tissues.
• The viruses are drawn as a many-sided protein shell (a capsid) with spikes. For a real capsid, see the beak and feather disease virus, which we solved with Jade Forwood’s group: a ball of 60 copies of one protein.
• The boost. Macrophages find trouble by following chemokines, chemical signals released at sites of infection (chemotaxis).
• The torn vessel wall. Platelets rush to plug the gap, the first step of haemostasis; threads of fibrin then turn the plug into a clot. A clot inside a vein is a thrombus.
• The colours. Red blood cells carry oxygen on haemoglobin. Oxygen-rich blood in the arteries is bright red; blood in the veins carries less oxygen and is darker red (not blue).
• The liver. Blood from the gut reaches the liver through the hepatic portal vein, bringing nutrients, some gut bacteria and ammonia, which the liver turns into urea in the urea cycle. The liver has its own resident macrophages, the Kupffer cells. Plasmodium, the parasite that causes malaria, infects liver cells before it attacks red blood cells.
• Reinforcements. Now and then fresh monocytes arrive from the bone marrow, where all blood cells are made (haematopoiesis). Catch them to recover.

Where the game cuts corners

• White blood cells don’t race along with the blood, steering round obstacles. They roll along the vessel wall, stop where there’s trouble and squeeze out into the tissue (leukocyte extravasation). The racing is done by the blood itself, which goes all the way round the body in about a minute.
• A macrophage doesn’t shoot its proteases. It swallows the microbe first and digests it inside, in the phagosome, so the enzymes don’t damage the tissue around it. Far less exciting to watch, much better for the tissue.