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Is Bigger Better? Understanding How Body Size Shapes Immune Defenses

Speaker: Dr. Cynthia Downs, SUNY College of Environmental Science and Forestry 

Body mass is likely to affect the way organisms evolve, develop, and use immune defenses; however, comparative data on the scaling of immune defenses is rare. Downs et al. investigated how the antibacterial capacity of serum scaled with body mass among >160 species of terrestrial mammals spanning 7-orders of magnitude in size. They show that antibacterial activity in serum across mammals exhibits was mass-invariant. Still, the serum of large mammals is less hospitable to bacteria than would be predicted by their metabolic rates. Specifically, hypometric metabolic rates would predict that a large species should have disproportionately lower antibacterial capacity than small species, but body size is unrelated to killing capacity across species. In contrast, neutrophils, a white blood cell involved in early immune defenses, increased disproportionately with body size in >200 species of mammals. Combined, these results suggest that large and small mammals might utilize different strategies of immune defenses. Scaling of early, broadly-acting immune defenses provides novel perspectives on the ecology of host-pathogen interactions and on their co-evolutionary dynamics. These results have direct implications for effectively modeling the evolution of immune defenses and identifying potential reservoir hosts of zoonotic pathogens.