LivingWhy are women better resistant to viruses?

Why are women better resistant to viruses?

According to the statistics provided by the Ministry of Health, the ratio between men and women infected by SARS-CoV-2 is 40-60. When considering the number of deaths, the relationship is reversed (60-40).

A more detailed analysis reveals that, of the total number of men affected by the virus, just over half (51%) required hospitalization and 5.4% of the total died. On the other hand, 31% of the women who tested positive required hospitalization and 1.8% died.

These differences between the sexes in virus infection are not new. The analysis of the evolution of the human immunodeficiency virus (HIV), which causes AIDS, among infected men and women, confirms that women have a lower viral load in their blood than men. The main reservoir of this virus is the lymphatic tissue (present in the lymph nodes, spleen, small intestine and lungs, among other organs) and from this it can pass into the blood, making it different from viral load among men and women could be related to a greater ability of women to eliminate it from the blood.

Other well-documented cases are hepatitis B and C viruses, which establish chronic infections and can cause fibrosis, cirrhosis, and hepatocellular carcinoma. This carcinoma develops more in men than in women in a ratio of 7 to 1.

Also herpes simplex viruses (HSV) 1 and 2, which can cause cold or genital herpes, respectively, have a different response in women than in men. They develop a higher level of specific IgG and IgM antibodies and, therefore, are more protected against potential neurological damage and viral reactivation.

Although these gender variations have not been observed for the common flu, they do exist for many other viruses. In general it can be stated that women respond more effectively to viral infections.


Differences in the immune and endocrine systems

What is the reason for these differences? The most commonly accepted hypothesis at present resides in the different behavior of the immune and endocrine systems, closely related.

Sex hormones have opposite effects when a virus attacks cells: estradiol, the main female hormone, has a potentiating effect on the immune response. Testosterone, the main male hormone, is mostly suppressive.

In general, the adaptive immune response –mediated by antigen-presenting cells and CD4 + T lymphocytes, leading to the production of antiviral antibodies and specific cytotoxic CD8 + T lymphocytes– is more intense in women.

After having passed a viral infection, the immune system returns to its homeostatic state. Still, baseline immune responses are much higher in women than in men. This is the reason why women are more prone to developing immunopathological diseases, while men are at risk for persistent viral infections more often.

But this hypothesis based on the most efficient, intense and prolonged immune response of women does not always explain all the scenarios in which we find ourselves in a viral infection. Sometimes, and following the guidelines of chance and necessity proposed by the biologist and winner of the Nobel Prize in Physiology or Medicine Jacques Monod, it seems that the virus is the one that decides, if this verb can be used for an infectious agent in the limit of what is considered alive.

Research carried out at the Royal Holloway University in London and published in Nature Communications proposes that it is the viruses themselves that decide to respect women more. Men and women can transmit viruses from person to person, but only women can do so vertically to offspring. This transmission to the offspring makes it of little advantage for the viruses to establish a lethal infection in the woman, since it would stop their spread.

The authors observed that HTLV-1, a virus associated with some types of human leukemia discovered by Robert Gallo, affects Japanese women unevenly compared to Caribbean women. They concluded that pregnancy and lactation could be the mechanism responsible for the differences. Women with longer lactation periods, in this case the Japanese, had a higher rate of vertical transmission of the virus, but in return the virus was less virulent for them.

It is not clear how viruses alter their virulence based on gender, but once the fundamentals of this mechanism are known, viruses could be tricked into being less virulent. They have undoubtedly contributed to consolidating a new medical discipline that, for 20 years, has been known as gender medicine.

Vicente Pallas Benet, CSIC research professor (Molecular Virology of Plants, Institute of Molecular and Cellular Biology of Plants IBMCP) and Javier Buesa Gómez, professor of Microbiology (University of Valencia)

This article was originally published on The Conversation. Read the original.

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