Molecular biologists at the Weizmann Institute of Science and colleagues have conducted a “census” of all the immune cells in the human body to fill a long-standing gap in scientific knowledge. And we found out how much the immune system weighs and takes up on average in our body. And also how its cells are distributed among different tissues and organs. The study was carried out on three types of standard human bodies – male, female and child. Scientists shared their results in an article published in the Proceedings of the National Academy of Sciences.

The human immune system is a vast and complex network of cells that are responsible for the critical processes of recognizing threats and protecting the body from them. It is known that there are many types of immune cells and they are located in different parts of the body; most have been identified by scientists. However, a general “census” of these cells has not yet been carried out: it is unclear how many cells of each type are in the average human body, what is their total mass and volume.

To find out, the authors of the new study systematized all available information about the different types of immune cells and cell mass. According to the scientists, most of the data in the scientific literature – to estimate the number of cells per gram of tissue mass – were not separated by sex and age.

“We used a standard reference person, historically characterized as a healthy male between 20 and 30 years of age, weighing 73 kilograms and height 176 centimeters. And then they extrapolated the results to other segments of the population: for adult women weighing 60 kilograms and a reference child of ten years old and weighing 32 kilograms,” the study says.

According to the results of calculations, it turned out that the average adult male has immunity of approximately 1.8 trillion cells, which weigh about 1.2 kilograms. An adult woman has approximately 1.5 trillion immune cells, her immunity weighs one kilogram. In children aged about ten years, the immune system already consists of a trillion cells and weighs 600 grams.

The researchers also discovered an interesting fact about the number and distribution of immune cells. It turned out that the most common of them, which make up 40% of the entire population, are lymphocytes, while weighing only 15% of the total mass. The proportions are approximately the same for neutrophils. But macrophages – only 10% of the population of immune cells – are so large that they account for 50% of the total cellular mass of our immunity.