Blood is not merely a means of transportation, it performs other important functions. Flowing through the body’s vessels, the blood in the lungs and intestines comes into close contact with the environment. Both the lungs and especially the intestines are the ‘dirtiest’ places in the organism. It is no wonder that in these sites bacteria can easily penetrate the blood. Why should this not happen? Blood is an excellent nutrient medium rich in oxygen. Were it not for the watchful and merciless guards at the gate, the course of the organism’s life would turn into the path of its death.
Guards proved readily available. At the dawn of life all the cells of the organism were capable of capturing and digesting particles of food. At about the same time organisms provided themselves with mobile cells, very similar to modern amoebae. They did not sit idly by and wait for the flow of fluid to supply them with something palatable, but continuously sought their “daily bread”. These wandering hunters, which at the very outset waged war against microbes invading the organism, became known as leucocytes.
Leucocytes are the largest cells in man’s blood, varying in size from 8 to 20 microns. These white-smocked sanitarians in our organism continued for a long time to take an active part in digestion and they still perform that function in modern amphibians. It is not surprising that there are large numbers of leucocytes in lower animals. In one cubic millimetre of fish blood there are as many as 80 thousand leucocytes, ten times as many as in a healthy human.
It takes very many leucocytes to combat pathogenic microbes successfully, and the organism produces large quantities of them. However, it has proved very difficult to determine their life-span. The leucocytes are of course ‘warriors’ and as such they probably never live to old age but are killed off in battle, that is in the fight for health. This may explain why under different experimental conditions in different animals leucocytes possessed life-spans ranging from twenty-three minutes to fifteen days.
More accurate data have only been obtained for lymphocytes, one of the types of white blood corpuscles in our body. Their life is ten to twelve hours, which means that the organism completely renews its stock of lymphocytes at least twice a day.