Creative Minds: Does Human Immunity Change with the Seasons?

Micaela Martinez

Micaela Martinez

It’s an inescapable conclusion from the book of Ecclesiastes that’s become part of popular culture thanks to folk legends Pete Seeger and The Byrds: “To everything (turn, turn, turn), there is a season.” That’s certainly true of viral outbreaks, from the flu-causing influenza virus peaking each year in the winter to polio outbreaks often rising in the summer. What fascinates Micaela Martinez is, while those seasonal patterns of infection have been recognized for decades, nobody really knows why they occur.

Martinez, an infectious disease ecologist at Princeton University, Princeton, NJ, thinks colder weather conditions and the tendency for humans to stay together indoors in winter surely play a role. But she also thinks an important part of the answer might be found in a place most hadn’t thought to look: seasonal changes in the human immune system. Martinez recently received an NIH Director’s 2016 Early Independence Award to explore fluctuations in the body’s biological rhythms over the course of the year and their potential influence on our health.

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Birth Year Predicts Bird Flu Risk

Incidence of Avian Flu vs. Year of Birth

Caption: Birth years of people in China who contracted H7N9 avian flu from 1997-2015 (left); birth years of people in Cambodia, China, Egypt, Indonesia, Thailand, and Vietnam who contracted H5N1 avian flu from 1997-2015 (right).
Source: Adapted from Science. 2016 Nov 11;354(6313):722-726.

You probably can’t remember the first time you came down with the flu as a kid. But new evidence indicates that the human immune system never forgets its first encounters with an influenza virus, possibly even using that immunological “memory” to protect against future infections by novel strains of avian influenza, or bird flu.

In a study that looked at cases of bird flu in six countries in Asia and the Middle East between 1997 and 2015, an NIH-supported research team found that people born before 1968 were at lower risk of becoming seriously ill or dying from the H5N1 strain of the bird flu virus than were those born afterwards [1]. Just the opposite was true of another emerging strain of bird flu. People born before 1968 were at greater risk of becoming seriously ill or dying of H7N9, while those born after that date were more often protected.

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Taking a Snapshot of the Human Immune System

There are numerous tests to gauge the health of your heart. But no such widely accepted test exists for the many parts of the immune system. How can we tell if the immune system is strong or weak? Or quantify how badly it’s malfunctioning when we suffer from asthma, allergies, or arthritis?

A team led by scientists at Stanford University has taken the first steps toward creating such a test—by taking “snapshots” of the immune system.

Before we talk about what they did, let me review how the immune system protects us against disease. The innate immune system is like a standing army that defends us against invading microbes. But the innate system has no memory. It doesn’t recognize the invaders more quickly if they return. This is the job of the adaptive immune system—B and T cells. These cells not only remember invaders; they’re able to adapt their weapons—antibodies and T-cell receptors—to make them more effective. Think of them as the Special Forces. Continue reading

How Influenza Pandemics Occur

Credit: National Institute of Allergy and Infectious Diseases, NIH

Flu season is upon us! Check out this NIH video to see how these pandemics emerge and spread new flu viruses around the globe.