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How much does your immune system weigh? Now we have an answer

Your immune system could weigh 1 to 1.2 kilograms and contain 1.8 trillion cells.

“The immune system is a complex system comprising many different cell types with essential functions,” says Ron Milo at the Weizmann Institute of Science in Israel.

Lymphocytes, for example, are a type of white blood cell that produce antibodies to neutralise viruses and other pathogens. Mast cells, another component of the immune system, control the body’s inflammatory response to injury.

To get a better insight into what constitutes a person’s immune system, Milo and his colleagues took a census of all its cells. Using measurements from past research, they estimated how many immune cells were in different tissue types around the body.

They combined this with laboratory analyses of samples of every tissue type, which they collected from several men and women of approximately the same age, as well as children, all aged 10 years old. None of the participants had any known health conditions.

The team found that a 73-kilogram man who was aged between 20 and 30 years old would have around 1.8 trillion immune cells that collectively weigh 1.2 kg. A 60 kg woman of the same age has 1.5 trillion immune cells, weighing 1 kg. The cell number declines to 1 trillion for a 10-year-old child, whose immune system weighs around 0.6 kg.

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Across both sexes, and regardless of whether the individual is a child or an adult, lymphocytes and neutrophils, another type of white blood cell, each make up 40 per cent of all immune cells and 15 per cent of their immune system’s entire mass. These cells reside primarily in bone marrow, lymph nodes and the spleen.

“Despite being commonly referred to as white blood cells, most of these cells primarily dwell in the bone marrow and lymphatic system,” says Milo. “Only a tiny fraction circulates in the bloodstream at any given moment.”

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Macrophages, which engulf pathogens, are the heftiest immune cells and contribute almost half of their total mass, despite only making up 10 per cent of the immune system’s cells.

By mapping the immune system, Milo hopes we can better understand how our bodies fend off infections and various medical conditions. “It can facilitate the creation of quantitative infection models, contributing to progress towards effective clinical treatments,” he says.

Journal reference:

Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.2308511120

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