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Study: Pregnancy causes surprising changes in how the immune system responds to the flu

When pregnant women get influenza, they tend to get really sick. Flu complications such as pneumonia are more common in pregnant women than other healthy young adults, and their risk of death from flu is higher, too.

Until now, doctors have ascribed the problem to the fact that the immune system is tamped down by pregnancy, a protective mechanism that keeps the woman's body from rejecting her fetus. But a new Stanford study, the first ever to directly examine how a pregnant woman's immune cells respond to flu viruses, found something unexpected: Instead of responding sluggishly, immune cells from pregnant women actually over-react to the flu. From our press release about the paper, which appears today in the Proceedings of the National Academy of Sciences:

“We were surprised by the overall finding,” said Catherine Blish, MD, PhD, assistant professor of infectious diseases and the study’s senior author. “We now understand that severe influenza in pregnancy is a hyperinflammatory disease rather than a state of immunodeficiency. This means that treatment of flu in pregnancy might have more to do with modulating the immune response than worrying about viral replication.”

In the lab, Blish's team incubated immune cells obtained from pregnant and nonpregnant women's blood samples with different strains of flu virus, including the H1N1 flu that caused the 2009 pandemic and also a less virulent strain of seasonal influenza. The responses they observed could help explain why flu, especially pandemic H1N1 flu, causes pneumonia in many pregnant patients:

Pregnancy enhanced the immune response to H1N1 of two types of white blood cells: natural killer and T cells. Compared with the same cells from nonpregnant women, H1N1 caused pregnant women’s NK and T cells to produce more cytokines and chemokines, molecules that help attract other immune cells to the site of an infection.

“If the chemokine levels are too high, that can bring in too many immune cells,” Blish said. “That’s a bad thing in a lung where you need air space.”

Why would influenza break the rules of how the immune system works in pregnancy? Blish thinks there's a clue in the fact that the flu produces a fourfold increase in an expectant woman's risk of delivering her baby prematurely. “I wonder if this is an inflammatory pathway that is normally activated later in pregnancy to prepare the body for birth, but that flu happens to overlap with the pathway and aberrantly activates it too early,” she said.

The research is a good reminder that flu season is just around the corner, and it's time to start thinking about getting a flu shot, especially if you are pregnant or planning a pregnancy.

Previously: Text message reminders shown effective in boosting flu shot rates in pregnant women, Ask Stanford Med: Answers to your questions about seasonal influenza and Flu shots for moms may help prevent babies from being born too small
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