Size Matters In Primates' Immune Response

Empirical data on the relationship between body mass and immune defense is limited. The metabolic theory of ecology predicts that larger organisms would have weaker immune responses, but recent studies have suggested that the opposite may be true, leading to the “safety factor hypothesis,” which proposes that larger organisms have evolved stronger immune defenses because they carry greater risks of exposure to pathogens and parasites. In the new study, Emily and team simulated sepsis by exposing blood from nine primate species (the largest being Homo sapiens, the smallest being the grey mouse lemur) to a bacterial lipopolysaccharide (LPS), measuring the relative expression of immune and other genes using RNAseq, and fitting phylogenetic models to determine how gene expression related to body mass. The researchers saw a positive association in the expression of innate immune genes, such that large primates had a disproportionately greater increase in gene expression of immune genes compared to small primates—in other words, the expression of inflammatory genes changed with body size when they were fighting an infection. This “hypermetric” immune gene expression appears to support the safety factor hypothesis, though this pattern may represent a balanced evolutionary mechanism to compensate for lower per-transcript immunological effectiveness. The study contributes to the growing body of immune allometry research, highlighting its importance in understanding the complex interplay between body size and immunity over evolutionary timescales.
July 12. 2024