Humans ability to defend ourselves against pathogens and cancer depends on a type of white blood cell called a T cell. This is not only true for humans, but for all vertebrates—animals with backbones. T cells recognize invading pathogens or abnormal tumor cells using a surface receptor called the T cell receptor (TCR). For the past 25 years, researchers knew of two kinds of TCR, called alpha/beta TCR and gamma/delta TCR, present in all the vertebrates. They were thought to be the only two kinds.

Several years ago, while investigating the immune systems of marsupial mammals such as the opossum, UNM Professor of Biology Rob Miller's laboratory discovered a third type of TCR that was named TCRmu. This particular receptor is found in all species of marsupials, such as kangaroos and koalas, but absent from all placental mammals such as humans, mice, cows and whales.

In a paper published this month in the Journal of Immunology, Xinxin Wang, UNM Ph.D. candidate, in collaboration with Research Assistant Professor Zuly Parra and Professor Miller, all from the Center For Evolutionary and Theoretical Immunology (CETI), reported finding TCRmu in the duckbill platypus. The platypus is a monotreme, an ancient lineage of mammals native to Australia, which lay eggs rather than give birth to live young. This discovery is significant in that the genetics of platypus TCRmu has provided insight into how this TCR evolved in mammals. More importantly, TCRmu's presence in both monotremes and marsupials indicates that it was present in a common ancestor of all living mammals, including humans, but was lost early in the placental lineage.

"Why placental mammals threw TCRmu away isn't known, and we won't be able to answer that question until we have a better understanding of what the T cells that use this receptor are doing in marsupials and monotremes," says Miller. The Center specializes in collaborative research on the evolution and diversification of immune systems across all organisms.

The research is published in the "In This Issue" section of The Journal of Immunology. "In This Issue" highlights articles that are among the top 10 percent of articles published in the journal. To read the abstract visit: Platypus TCRμ Provides Insight.

For more information on the research contact Jennifer Kavka, UNM Center for Evolutionary and Theoretical Immunology, (505) 277-5508 or e-mail,

Media Contact: Steve Carr (505) 277-1821; e-mail: