The fourth and recently discovered weapon, or ammunition, of the blood is the opsonins. Wright and Douglas in London in 1903 coined the word, which comes from the Latin _opsono_: “I cook for the table,” “I prepare pabulum for.” This is precisely what the opsonins do in the blood. They manifest this beneficial activity when invading disease germs appear. They attract white blood cells to the germs and make the bacteria more eatable for the cells. They are appetizers for the white blood cells; or sauces, which help the white blood cells to eat more of the bacteria than they could do without this spur to their hunger. Wright and Douglas demonstrated beyond peradventure the ability of the white blood cells to eat a larger number of bacteria when the latter are soaked in opsonins. They also showed that this opsonic sauce, or appetizer, which stimulates the blessed hunger of the white blood cells for disease bacteria, could be artificially produced, and hypodermically introduced into a patient’s blood, thus increasing that blood’s power of defense by raising the quantity of opsonins. They also worked out a practical laboratory technique by means of which the opsonins can be measured, or counted, with a considerable degree of exactitude, thereby making it possible to estimate within limits of accuracy any one’s ability to resist bacterial invasions. If the blood is rich in opsonins, its power to fight disease is strong. Opsonins are now inoculated into the blood at several institutions, notably McGill University in Montreal, and at the Battle Creek Sanitarium. HOW THE WHITE CELLS DO THEIR WORK