Number of human immune cells collected

Graphical representation of the distribution of immune cells among the organs in the human body

Estimates of human immune cell numbers by cell type and tissue, grouped by primary systems. © Ron Sender, created with BioRender.com

The human body is made up of trillions of cells that vary greatly in form and function. A blood cell, for example, is much smaller and more compact than a muscle or nerve cell. Just a few weeks ago, researchers recorded the average number of all cells found in the human body. But how many of these cells belong to the immune system? And which immune cell types can be found where in the body? Another research team has now discovered this. Accordingly, the immune system of a 73 kilogram man consists of around 1.8 trillion cells, which together weigh around 1.2 kilograms. The immune cell types of lymphocytes and neutrophils make up the largest proportion of this.

All living things, including us humans, are made up of cells, the basic units of life. In organisms like humans, which consist of several different cells, the individual cell types are specialized depending on the tissue and function and therefore differ in their shape, size or metabolism. A few weeks ago, a research team determined how many cells there are in the human body and how the different cell types differ in terms of cell size, number of cells and mass fraction. Now other researchers have taken a closer look at the cells of our immune system and analyzed their composition and distribution in our body. Our immune system also consists of different cell types that are specialized for their respective tasks in defending against pathogens. Together they form a network that includes various tissues and organs, such as the lymph nodes. However, it was still unclear how exactly the immune cells were distributed.

For their study, researchers led by Ron Sender from the Weizmann Institute of Science in Israel evaluated various data and combined them to determine the number, mass and distribution of cells in the human immune system. The team evaluated the existing specialist literature and analyzed data from, among other things, medical imaging of tissues, cell counting of individual cell types from different tissues and organs, and cell detection based on epigenetics. Because these data were collected using different methods and definitions, they were not always directly comparable. Sender's team used statistical analyzes and meta-techniques to generate comparable data and get an overall picture of our immune system.

1.2 kilograms of immune cells in men

Their analysis showed that an adult man's immune system consists of around 1.8 trillion cells. For his average body weight of 73 kilograms, there are 1.2 kilograms of immune cells. A woman weighing 60 kilograms consists of 1.5 trillion immune cells, which together weigh around one kilogram. In a 10-year-old child who weighs 32 kilograms, there are about a trillion immune cells weighing 600 grams.

Graphical representation of the distribution of immune cells among the organs in the human body
Estimates of the mass of all human immune cells by cell type and tissue, grouped by primary systems. © Ron Sender, created with BioRender.com

Since the individual immune cells are different sizes and weights, they have different proportions of the cell number and cell weight of the immune system. The scientists report that the largest proportion is made up of lymphocytes, which are mainly located in the lymph nodes and the spleen and include T cells, B cells, natural killer cells and plasma cells. These immune cells make up 40 percent of the cells and 15 percent of the cell mass of the immune system. Neutrophil cells, which are mainly found in the bone marrow, are similarly common in our immune system. According to the study, the myeloid immune cells, which primarily include the macrophages found throughout the body, only make up ten percent of the cell number but 50 percent of the mass of the immune system because they are significantly larger and around ten times heavier than the other immune cells.

Surprise in the gut

With regard to the distribution of immune cells in our body, the researchers surprisingly discovered that the intestine does not contain most of the body's immune cells, as previously assumed. On the contrary, it only contains around three percent of our immune cells. The intestine is primarily home to lymphocytes, but only around five percent of all lymphocytes in our immune system can be found in the intestine. According to the study, similarly few immune cells are found in our skin, our lungs and our blood (two to four percent of all immune cells each). Instead, most of the immune cells in terms of number and weight are located in the lymphatic system and in the bone marrow, each accounting for around 40 percent of our immune cells and around 30 percent of their mass. These organs therefore consist mainly of immune cells and are also the most densely packed with them.

The researchers report that the distribution of immune cells among our organs and their differentiation according to cell type is largely independent of gender. However, as we age and become ill, the composition of our immune system changes. Further studies must show what exactly happens with aging and long-term illnesses such as cancer and how serious the changes are to our immune system. Overall, the results represent an approximation based on estimates and reanalyses of existing data. A complete count of which cells occur in the human immune system, how often and where, has not yet been carried out. However, Sender and his colleagues hope that their findings will help us better understand our immune system and develop quantitative models of the human immune system.

Source: Ron Sender (Weizmann Institute of Science) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2308511120

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