Body size and the immune system

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Can body size impact how an immune system fights disease? Does an elephant have a different immune system than a mouse?

According to USF College of Public Health (COPH) professor Dr. Lynn (Marty) Martin and colleagues, it’s a subject that’s been virtually unstudied.

Until now.

Martin and others recently published the article “The effects of body mass on immune cell concentrations of terrestrial mammals” in the journal The American Naturalist. The journal posted the article online in September.

The group studied whether body size was related to concentrations of two important immune cell types (lymphocytes and neutrophils) in the blood of hundreds of mammalian species―from tiny Jamaican fruit bats to elephants and giant killer whales.

The COPH’s Marty Martin, PhD, and fellow researchers discovered that big mammals have very different immune systems than small ones. (Google Images)

And what they found was that size does, indeed, matter.

While the amount of lymphocytes―immune cells that kill viruses and help the body make antibodies―remained the same per liter of blood in big and small mammals, neutrophils, which kill or engulf bacteria and other infectious agents, were disproportionately more numerous in bigger species.

“One possibility for why lymphocytes remained the same,” said Martin, who specializes in global health and disease ecology, “is because they are a very diverse class of cells, meaning they perform many functions. We need better tools to count different types of lymphocytes before we’re likely to see any patterns.”

On the other hand, Martin added, bigger mammals have disproportionately more neutrophils than their smaller counterparts, perhaps due to cell replication.

“Because bacteria divide at fast rates―faster than the neutrophils that fight them―big mammals, with their increased size, might be at risk for more infection. As such, bigger mammals need more neutrophils than smaller ones to fight the spread of infections.”

Neutrophil cells. (Google Images)

While the group only studied mammals, Martin says the research results should hold in other species, and, in fact, preliminary work in birds is revealing similar patterns. These discoveries could also help improve human health.

“Specifically,” said Martin, “the research tells us something about how being bigger leads us to have a different immune response than the main species we study in immunology―lab mice and rats. It might even enable us to use health interventions more effectively by adjusting our approaches based on human versus mice size-differentials.”

Story by Donna Campisano, USF College of Public Health