Did you know that women get autoimmune diseases, such as multiple sclerosis, lupus and rheumatoid arthritis eight times more than men do?
On the other hand, women have a smaller risk of getting non-reproductive cancers such as melanoma, colon, kidney and lung cancer.
While there are some exciting developments in cancer treatments, such as immunotherapies, research is showing that women are responding more favourably than men to this type of intervention.
So why is there such a big difference between women and men when it comes to human diseases?
The ‘Pregnancy Compensation Hypothesis’
An interdisciplinary team of scientists at Arizona State University have a new hypothesis, called ‘The Pregnancy Compensation Hypothesis’, to explain the differences between women and men when it comes to human diseases.
“Basically, women’s immune systems evolved to facilitate their survival during the presence of an immunologically invasive placenta and pregnancy and compensate so they could also survive the assault of parasites and pathogens. But now, in modern, industrialised societies, women are not pregnant all the time, so they don’t have a placenta pushing back against the immune system,” says Melissa Wilson, assistant professor with ASU’s School of Life Sciences.
She says that the changes in women’s reproductive ecology exacerbate the increased risk of autoimmune disease because immune surveillance is heightened; while at the same time we see a reduction in some diseases, like cancer.
The impact of modern life
Another factor that may exacerbate this situation is living a modern-day, urban lifestyle.
In industrialised communities, autoimmune diseases appear to occur at a much higher rate than in non-industrialised populations.
The researchers believe that the human immune system evolved expecting a given load of parasites. In the modern environment, exposure to those parasites has diminished so the immune system has fewer foreign targets. With this reduced load, the immune system attacks ‘self’.
“There is a mismatch between the ancestral environment humans were adapted to, and the industrialised environment many people currently live in. In terms of an evolutionary timescale, our environment has changed incredibly fast,” said Angela Garcia, also a postdoctoral research fellow with the centre.
“We have also shifted from an active lifestyle to a sedentary one. We now have an overabundance of calories available, which potentially allows us to maintain excessive levels of hormones, including the female hormone estradiol. Maintaining such high levels of hormones may increase the chance of triggering autoimmune diseases,” she said.
Cancer is different in men and women, so treatments should be too
Heini Natri, the lead author of the study and a postdoctoral scholar with the ASU Centre for Evolution and Medicine, says that, because the immune system varies between the sexes, it should be considered when developing immunotherapies and other treatments.
“We think the Pregnancy Compensation Hypothesis can explain why there’s a big sex difference in these diseases. Going forward, understanding the evolutionary origin of the sex bias in these diseases can help us better understand the mechanisms and particular pieces of the immune system we can target,” said Natri.
“Our goal is to actually make treatments better for everyone. We are realising that cancer is different in men and women. In the study of most cancers and other diseases, and so far in the development of cancer treatments, that has not really been taken into account.”