An organism’s phenotype is the result of its genotype and its environment. Teasing apart the relative importance of these factors in determining phenotype is a difficult task. However, monozygotic (i.e. identical) twins offer a natural experiment to test the contributions of genes (‘nature’) and environment (‘nurture’) to phenotype.
In their 2015 Cell paper Brodin et al. measured 204 immunological parameters in 210 sets of healthy twins between 8 and 82 years old. They found that variation in immunity between twins was too great to be explained by variation in their genomes. This suggests that the environment plays a larger role than genotype in determining an individual’s immune system phenotype.
Our results show that these functional units of immunity vary across individuals primarily as a consequence of non-heritable factors, with a generally limited influence of heritable ones.
Brodin et al. also found that young twins were more similar to their sibling than old twins were to theirs, suggesting a divergence of immune systems over time as twins are potentially exposed to different environments than their sibling. This also supports the hypothesis that environment influences immunity more than genotype.
the immune system of healthy individuals is very much shaped by the environment and most likely by the many different microbes that an individual encounters in their lifetime.
Brodin P, Jojic V, Gao T, Bhattachatya S, Lopez Angel CJ, Furman D, Shen-Orr S et al. (2015) Variation in the Human Immune System Is Largely Driven by Non-Heritable Influences. Cell (160) 37-47. DOI: 10.1016/j.cell.2014.12.020