There are two basic arms of the immune system: innate and adaptive. 

Innate immunity is the first line of defence, staffed by general purpose, pathogen-killing cells such as neutrophils and macrophages. These are the early responders to an invasion. 

The adaptive side is more targeted and slower, reacting to specific pathogens with precision weapons such as T-cells, B-cells and antibodies. 

The adaptive arm also provides immune memory, which prevents you from getting certain diseases twice. 

Cells called memory B-cells recognise the pathogen and trigger a swift and ruthless response if it invades again. 

Some viruses – notably flu – can mutate to evade immune memory. We don’t know yet if the new coronavirus does this.

DO SOME PEOPLE NATURALLY HAVE A STRONGER IMMUNE SYSTEM? 

Even among people with a fully functioning immune system there are significant differences in how well it works. 

In 2018, the Milieu Intérieur Consortium based at the Pasteur Institute in France scrutinised circulating immune cells from 1000 men and women aged between 20 and 69.

They found major individual differences between people of different ages, which you would expect given that the immune system declines as we get older. But they also discovered that people of the same age can have very different immune systems, beyond variation in their so-called immune age. 

This is partly down to varying lifetime exposure to viruses and bacteria, which can radically alter the composition of your adaptive immune system – the wing of your defences that produces antibodies targeted to attack threats. 

Genetics is also key. The Pasteur team found big disparities in the composition of people’s innate immune systems, the generalised first line of defence, and these mapped onto differences in their genes. 

The significance of these genetic variations in immune response isn’t yet known, but it may be that some people are naturally better than others at dealing with certain threats.

Like a lot of things in life, immune strength is a genetic lottery. 

Beyond genetics, the team found that smokers have a much older immune profile than non-smokers of the same age. It’s unknown if this is reversible. But if you don’t want to prematurely age your immune system, it would be best not to smoke. 

Similar research on immune ageing by researchers led by Shai Shen-Orr at the Technion-Israel Institute of Technology suggests that the immune system can burn out. 

They tested the immune age of children living in Bangladesh, who generally have a heavy burden of infectious diseases and parasites, and found it was similar to that of young adults in California. 

The long-term clinical relevance of this is unclear, but it seems to undermine the adage that “what doesn’t kill you only makes you stronger”. 

That said, an underworked immune system also seems to be problematic: people who aren’t exposed to infections and parasites in childhood seem more susceptible to autoimmune disorders such as allergies later in life.