The days getting shorter and temperatures dropping always seems to be inevitably linked to various colds and other bugs going around. “Oh yes”, we say in response to someone’s malady, “there’s definitely something going around. Well, it is the season for it!” And no doubt about it, there are definite seasons for increase in common illnesses, just ask any parent around school start! But why does this happen? Is there a universal drop in immunity? An increase in microbes? Overwhelm of the immune system? Let’s take a look at what’s going on in our bodies…
The Immune System: A Closer Look
Having an immune system is something we share with most other living creatures, with some being more complex than others. Immunity evolved alongside the multitude of viruses, bacteria, and other pathogens that we are exposed to, in an arms race to fight them off as they continue to attack.
The immune system is a complex network of cells, organs, proteins and tissues that defend the body from outside invasion, by detecting unfamiliar substances and destroying them. We often think of our immune systems as the measures that stop us from getting ill in the first place, but it extends to also battling invading pathogens and storing records of those it has encountered to be able to launch a future attack much quicker.
Human immune systems have two different “departments”: the innate and the adaptive immune systems. The innate immune system is the more primitive system responsible for the initial prevention as well as broad scale destruction of any pathogens that make it into the body. This includes the barriers such as our skin, mucus membranes, and stomach acid, that stop entry of unwanted things. The innate immune system also provides the body’s first responders (like phagocytes and natural killer cells) as a general response. Should there be a breach, there are measures to immediately deal with and dispose of invaders, through a broad-measures solution that reacts to anything not identified as “self”.
The adaptive immune system is a more recent evolution, and is a much more specific and targeted than the innate one, as it learns to recognise foreign materials, responding to very specific molecules, and launches a targeted attack before such a pathogen can spread and cause damage. This is what vaccination is based on: cells like lymphocytes in the adaptive immune system learns to recognise a virus through a small, inert presence through the vaccination, creating a stored antibody that acts like a quick-reference. The whole immune system can then quickly deal with the same pathogen again should the body become infected with a live virus, because it recognises it from before.
Cold and Flu Season
While some unlucky souls come down with a cold anytime of year, there are definite seasons for illnesses. We know that there are seasonal cycles of infectious diseases that vary depending on type of pathogen and part of the world it’s found. For instance, flu season is both monitored and predicted every year by government bodies, with influenza infections peaking mid-winter.
The seasonal cycles of infectious diseases are tied to things like atmospheric conditions, prevalence of pathogens, and behaviours of hosts. Respiratory infections peak in the winter months in temperature regions like the UK, while Malaria peaks after wet periods in the equatorial countries. Colds are called colds for the fairly obvious reason that they show up when it gets cold. Low temperatures and high humidity are scientifically linked to common colds and respiratory tract infections. Some viruses, like COVID and hepatitis, stay active outside a host for longer in lower temperatures. This might be one reason we see increases in infections as temperatures drop - pathogens have a longer shelf life so you can pick one up from a surface for much longer after someone sneezed or coughed on it.
While external factors influence the lifecycles and spread of pathogens, our defence against these infections are also seasonal, an evolutionary measure to counter these attacks.
Are humans seasonal?
So we know that germs have their own seasons – but what happens to our bodies and immunity during the seasonal change?
The conditional seasons we experience in temperate climates are caused by the Earth’s tilt on its axis that means we get different amounts of sunlight and heat as the Earth rotates around the sun over the course of a year. Like most living things, humans also evolved around seasonality, interacting with other seasonal species. We follow rhythms of light, both when it comes to keeping track of our day but also our seasons. We release melatonin when it’s dark, which boosts sleep. People sleep longer in the winter than in the summer on average. A 2016 study also found that cognitive function peaked around summer solstice and hit a minimum at winter solstice. Our metabolism also slows down when our bodies register winter, an evolutionary response to harsher conditions - colder temps and less food means we had to ration our fat stores in the old days. Both our brains and our bodies slow down biologically when winter approaches, but what does our immune system do? Let’s take a look at seasonal immunity.
Seasonal Immunity
Humans didn’t evolve in a vacuum, but rather have been influenced by all our surroundings, including pathogens. Interestingly, this has also led to our immune systems bolstering up in winter-time, to try and get the upper hand on the incoming attacks and fend off illness.
Scientists have discovered that the cellular composition of our blood varies by season, with increased numbers of white blood cells, lymphocytes, monocytes, basophils, eosinophils, neutrophils and platelets in winter. This seems to be an evolved function, where the threshold to trigger an immune response is lowered in the winter in response to the barrage of pathogens we come across. In general, anti-inflammatory levels are lower, meaning inflammation is triggered much easier than during other times of year. This is useful in creating worse conditions for pathogens in our bodies as the white blood cells kill them off, but it also affects our own cells. Conditions like arthritis, where the immune system targets our joints and cause inflammation, tend to increase during the winter for this reason.
In 2015, a large study was conducted into the genomics of this immune bolstering, looking at more than 16,000 people globally. They wanted to find out if the expression of different genes changed during the seasons. Amazingly, they discovered that 23% of the total human genome shows seasonal differences in when they get expressed! About half of those were expressed more in the winter, and the other half in the summer (for people from temperate regions). They also discovered that the “winter” genomic profiles were opposite in Europe and Oceania, meaning that they are triggered during the time of actual winter where people are. This pattern also continued to line up with pathogen prevalence in the equatorial regions, where the immune genes were more active around the time of the wet season, when infection rates are also at their peak for localized pathogens like malaria. And as with any rule, there is always an exception: people in Iceland showed no variation in genes linked to immunity by time of year. Maybe because there is far less contrast between the seasons that far north?
A Struggling Immune System is a Pressure Cooker for Our Health
We’ve looked at both sides of the flu season arms race – pathogens increase their attacks, but in return we’ve evolved more proactive immune systems during winter. So why do we still get ill?
Although we are fortunate enough to for the most part be buffered from the winter harshness our ancestors had to endure, our social evolution and behaviours throw some spanners in the works for our hardworking immune systems!
With winter approaching, some animals slow down and hibernate for the winter, preserving their resources. Our biology is also trained to hunker down, but in reality things get more hectic than ever. As kids go back to school in the autumn the increased contact leads to easy transmission of pathogens. We basically create our own micro-seasons that also impact how susceptible we are to disease!
This was scientifically studied in measles cases: patterns in childhood diseases reflect the regular increase in contact rates during school terms. On top of our social seasons, our drift away from the biological slowdown is exacerbated by our lifestyles.
Environmental disturbances of our molecular body clocks is bad for our health, which may explain the increasing complex disease burden in industrialised countries where we don’t follow our body clocks. Stress levels rise as we approach the holiday season, with endless to-do tasks and preparations. Stress and burnout directly impact the immune system, as the prolonged fight-or-flight response of cortisol release demand resources be funneled into survival rather than immunity. This redistribution of resources takes away from the immune response and makes us more susceptible to disease. Coming down with an illness in the first instance also lowers overall immunity and leaves the door open to further attacks while your immune cells are vulnerable.
While our immune systems have evolved to bolster themselves during the colder months, social behaviours, stress, and environmental factors can throw that finely tuned balance off course. Understanding how immunity changes with the seasons (and how our actions can support or hinder it!) gives us the power to reduce our risk of illness. Prioritising rest, managing stress, maintaining good hygiene and keeping our bodies nourished and active can help our immune systems do what they were designed to do. In the end, winter’s challenges are a reminder that we’re seasonal creatures who need to take a leaf out of nature’s book and just rest for a while.
