The immune system is large and fascinating at the same time. If a wound occurs somewhere in the body, millions of immune cells are there in no time. The body’s own defense system comprises around 20 different cell types; they are located in various tissues or patrol the body – but how big is this immune army exactly? Scientists led by Ron Milo from the Weizmann Institute in Rehovot, Israel, wanted to know more about this. Came out a comprehensive measurement of the immune system, which the researchers published in the specialist journal PNAS present.
Accordingly, the immune system of an adult man weighing 73 kilograms consists of around 1.8 trillion cells. This is a considerable proportion of the body’s cells as a whole: in which there are, A recent study from Stanford found that 36 trillion cells, if he weighs around 70 kilograms and is 176 centimeters tall. People spend around five percent of their cells on their internal security. Together, the army of immune cells weighs 1.2 kilograms, the researchers write.
In addition to existing specialist literature, the Israeli scientists also used multiplex images of various tissues and direct cell analyzes for their immunological hit. Accordingly, the proportions of the various immune cells are distributed very differently: Lymphocytes, which are located primarily in the lymph nodes and the spleen and which, in addition to T and B cells, also include natural killer cells and plasma cells, make up 40 percent of all immune cells. However, because of their manageable size and weight, they only make up 15 percent of the mass of immune cells. The fraction of neutrophils, which reside primarily in the bone marrow and fight unwanted intruders with a corrosive cargo that they carry inside, is similarly large. The voracious macrophages, on the other hand, which sit in tissues throughout the body and wait for food in the form of sick or dangerous cells, make up ten percent of the immune cells, but make up around half of their mass.
Surprisingly few immune cells reside in the intestine
There are the most immune cells in the lymphatic system and bone marrow, around 700 billion each, the skin has 80 billion, the lungs have 70 billion, the liver and gastrointestinal tract have 50 billion each and the blood has 40 billion.
Previous studies have shown that in every milliliter of blood there are around five billion red blood cells and up to 1.5 million T cells. The new study confirms this magnitude. While the intestine was previously considered one of the most important locations of the immune system, the Israeli researchers only found around three percent of all the body’s immune cells there. Until now, immunologists had assumed that up to 70 percent of immune cells were located in the intestine. However, the immunological importance of the intestine remains great even according to the new analysis. Because around 70 percent of the important antibody-producing lymphocytes appear to be on the move there, as Ron Milo’s team admits.
The immune system is particularly important in the intestine, says Carsten Watzl, Secretary General of the German Society for Immunology and immunology professor in Dortmund. The intestine, with its huge surface area, is considered, along with the skin, to be the most important barrier to the outside world, from which hostile germs can constantly enter the body – especially in the intestine through food, which always poses the risk of infections. The task of the intestine is therefore complicated, says Watzl: While it has to fight off hostile pathogens and keep toxic food components away from the body, it also has to absorb the good things from food and tolerate the friendly bacteria on which the body depends.
However, the immune system is highly dynamic and its work is also influenced by other factors such as age and gender. For this reason, the researchers also measured the immune cells of a 60-kilogram woman and a ten-year-old child. At first glance there were no significant differences. It seems to have little to do with the distribution of cells in the body that the female immune system has greater power. The reason is that female sex hormones ensure that women produce more antibodies.