Powered flight has evolved several times in vertebrates and constrains physiology in ways that likely shape how animals cope with pathogens and parasites. Understanding how the evolution of flight may inform relationships between body size and immunity could shed light on the ability of some taxa to harbor many virulent viruses without showing clinical disease. In this study, we used an allometric framework to quantify the scaling relationships between body mass and the proportions of two types of white blood cells, lymphocytes and neutrophils/heterophils, across 60 bat species, 414 bird species, and 256 non-volant mammal species. To do this, we combine data from (1) field studies of Neotropical bats, (2) published literature on other wild bat species, and (3) zoo-captive birds and non-volant mammals. Using phylogenetically-informed Markov Chain Monte Carlo generalized linear mixed models (MCMCglmm), we show that lymphocyte and neutrophil proportions do not scale with body mass among bats. By contrast, larger birds and non-volant mammals have higher heter-/neutrophil proportions, respectively, than smaller species in these taxa. However, flying endotherms (bats and 400 bird species) have disproportionately higher neutr-/heterophil proportions compared to non-flying endotherms (mammals and 14 bird species). Taken together, these results imply some differences between this aspect of cellular immunity in bats compared to birds and non-volant mammals.