3. The immune system and natural selection

“Infections have been a huge cause of mortality in humans practically until just yesterday,” said Lluis Quintana-Murci, director of the Institut Pasteur in Paris. “Immune responses to them vary among individuals, and among populations, too,” he continued.

In 2016, his team published a revealing paper. They took defense cells from Africans and Europeans and exposed them to different bacterial and viral products. In studying their response, they saw that the African cells were three times more powerful. Hundreds of genes were expressed differently and changes in the genetic sequence –some inherited from the Neanderthals- explained most of these differences. Does this mean Africans have better defenses? Not necessarily. The hypothesis is that Europeans, as they were in contact with fewer pathogens, adapted by relaxing their defenses. And that could explain their lower rate of autoimmune diseases. It is a question of balancing risk and reward.

There are also individual differences. As explained Mihai G. Netea, head of the Experimental Medicine Laboratory at Radboud University Nijmegen Medical Center, in the Netherlands, “Some (non-immunodepressed) patients beat a Candida infection in just 1-2 days, while it takes others more than 10.”

In an attempt to explain this, “several consortia have been set up to study what is a healthy immune response and how it varies among individuals and populations,” explained Quintana-Murci. These groups will analyze genetic and epigenetic aspects —including the role of the microbiome— and will also do exhaustive interviews to determine hereditary influence and that of the environment.