Can ‘body clock’ govern immune response?
When microorganisms – such as bacteria or viruses – infect us, our immune system jumps into action. It's highly trained to sense and eliminate infections and clear up any damage caused by them.
It's typically assumed our immune systems work the exact same way regardless of whether an infection occurs during the day or at night. But research spanning over half a century now shows our bodies actually respond differently at day and night. The reason for this is our body clock, and the fact that each cell in the body, including our immune cells, can tell what time of day it is.
Our body clock has evolved over millions of years to help us survive. Every cell in the body has a collection of proteins that indicate the time depending on their levels. Knowing whether it's day or night means our body can adjust its functions and behaviours to the correct time.
Our body clock does this by generating 24 hour rhythms (also termed circadian rhythms) in how cells function. For example, our body clock ensures that we only produce melatonin as night falls, as this chemical makes us tired – signalling it's time for sleep.
Our immune system is composed of many different types of immune cells that are continually patrolling the body looking for evidence of infection or damage. But it is our body clock that determines where those cells are located at particular times of the day.
Broadly speaking, our immune cells migrate into tissues during the day and then circulate around the body at night. This circadian rhythm of immune cells may have evolved so that immune cells are directly located in tissues at a time when we are more likely to be infected, primed for attack.
At night, our immune cells circulate around the body and stop off at our lymph nodes. Here, they build up memory of what was encountered during the day – including any infections. This ensures they can respond better to the infection the next time they encounter it.
Given the body clock's control over our immune system, it's hardly surprising to learn that some research has shown that the time we're infected with a virus – such as influenza or hepatitis – can impact how sick we become.
Other research has also shown that the time we take our medicines can affect how well they work.
There's also an increasing body of evidence showing vaccines – which create an immune "memory" of a particular pathogen – are impacted by our body clock, and the time of day that a vaccine is administered.
For example, a 2016 randomised trial of over 250 adults aged 65 and older showed having the influenza vaccine in the morning (between 9am and 11am) resulted in a greater antibody response compared to those vaccinated in the afternoon (between 3pm and 5pm).
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