According to work from Broden et al. (2015), the variation in the human immune system is largely driven by non-heritable influences.
If there is little heritibility how can the genocide of native Americans have happened?
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If you are referring to the massive death toll among native Americans due to the epidemic disease outbreaks after first contact with European settlers, then the answer is rather straightforward (Science magazine News, 2016):
The immune system is a complex structure, built over a person’s life in response to environmental conditions. Antibodies, proteins that tag and attack viruses and bacteria, “remember” past invaders, allowing white blood cells to quickly respond during subsequent infections. Because different groups of people encounter different diseases—the European settlers had high exposure to smallpox, measles, and influenza thanks to close contact with livestock—they develop different antibodies.
Because the native Americans lacked exposure, they were highly susceptible to these new diseases.
Note, however, that their genetic make-up may have contributed as well (Science magazine News, 2016). A team of researchers compared skeletal remains of indigenous peoples (after being decimated by European disease) with their living descendants and found that:
...several immune-related gene variants that were rare among the living. For example, a variant of a gene known as a HLA-DQA1, which codes for proteins that sort healthy cells from invading viruses and bacteria, was found in nearly 100% of ancient individuals, but in only 36% of modern ones.
This finding suggests that the immune-related genes of the indigenous peoples were well-adapted to local diseases, but not to novel, European bugs causing smallpox and measles.
There was extensive pre columbian contact so the entire theory may be rejected. Example
The claim that smallpox killed all the natives originated from historians. There was never any science or evidence behind it. As the other answer showed modern natives actually have less of the purported genes. Biologists have no need to defend it.