Researchers at Tulane University, working with collaborators at Stanford University and the Centers for Disease Control and Prevention, may now have identified part of the answer.

A study published in Science Advances found that the world's largest family of bats carries two distinct copies of the genes involved in producing antibodies. These specialized proteins help the immune system identify and fight infections. Every other known mammal has only one such set of genes.

The finding gives scientists a new way to think about how immune systems evolved and how different animals respond to disease.

"We've never seen anything like this in a mammal before," said Hannah Frank, associate professor of ecology and evolutionary biology at Tulane University School of Science and Engineering and corresponding author of the study. "This completely changes our understanding of how mammalian immune systems can be organized and raises exciting new questions about why bats have been so evolutionarily successful and how they respond to viruses."

A Unique Antibody System in Vesper Bats

The discovery was made in vesper bats, a group that includes more than 500 species living on every continent except Antarctica. Scientists have long been interested in the extraordinary evolutionary success of these bats, and their unusual antibody system may provide part of the explanation.

Antibodies are Y-shaped proteins built from two heavy protein chains and two light protein chains. In humans and all other known mammals, the heavy chains are produced using a single set of genes.

Frank and her colleagues found that vesper bats are different. They have two separate heavy-chain gene systems, potentially giving them another way to create a wider variety of antibodies.

Researchers studying bat immunity have traditionally paid more attention to the innate immune system. Frank said the new findings show why the adaptive immune system -- the part responsible for producing antibodies -- also deserves closer study.

"We think this discovery is an important piece of the puzzle," Frank said. "It doesn't fully explain why bats are such effective viral reservoirs, but it reveals a level of immune variety we didn't know existed and gives us an entirely new direction to explore."

Why Bat Immunity Matters

Bats play important ecological roles as pollinators, seed dispersers and natural pest controllers. At the same time, they are natural hosts for many viruses, Frank said.

Learning how bats can live alongside those viruses without becoming ill could eventually help researchers better understand immune responses in other species. That knowledge may also contribute to improved strategies for reducing the risk of disease spillover.

"We've learned an enormous amount about immunity by studying humans and laboratory mice," Frank said. "But the natural world is far more varied than that. Every time we study a species that has evolved differently, we have the opportunity to discover something entirely new."