WHEN you are under the covers, sniffling from your third cold of the year, you might wonder whether your immune system is pulling its weight. It is a question we would all love to answer: is our immune system healthy or not? Now, a burgeoning field of immunology is attempting to do just that with tests that “grade” your immune system, the idea being that it could help people preempt problems and motivate them to make lifestyle changes to avoid ill health.

To understand how healthy your immune system really is, let alone improve it, first it is helpful to get to grips with what it actually does. Consider it your internal army, with different units coordinating to eliminate enemy invaders, all while avoiding collateral damage to your own tissues and constantly adapting to new challenges.

Like a real army, your immune system can also fall into disarray, with some units underperforming while others go rogue. This can cause major illnesses.

So how do you tell whether your army is doing its best? Let’s begin by considering the different components. The first line of defence comes from innate immune cells such as macrophages, which engulf bacteria and release chemicals that trigger inflammation, making cells less hospitable to pathogens.

Once a threat has been identified, a second line of defence called the adaptive immune system kicks in, with more specialised tools. B-cells, for example, form antibodies that bind to the unwanted newcomer and render it harmless. T-cells, meanwhile, include killer cells that attack infected cells, as well as helper cells that coordinate the activities of the other immune cells and aid the B-cells in producing antibodies. Regulatory T-cells are also on hand to call off the battle when the threat has passed, so the body can repair the damage.

Our biological warriors can be found lurking around the body. “They’re in the mucosal lining of our airways, the digestive tract and in our skin – anywhere things that could upset the body might get in,” says Jenna Macciochi, an immunologist at the University of Sussex, UK.

They are also aided by microorganisms in the body, known as the microbiome. “We often think about the microbiome in relation to the gut, but we have microbiota living all over our body,” says Macciochi.

When we are at peak health, these different components are perfectly balanced – but there are ways that things might go awry. You may lack the friendly microbes that protect your digestive system, meaning you are more susceptible to gut infections. A bad diet or stress can make you more sluggish at producing the correct immune cells to fight infection, and after you have recovered, you might lose the cells that “remember” the pathogen, should you be exposed again. Since the immune system can also spot and remove tumours, underactivity can render you more susceptible to cancer.

Other problems arise from overactivity, when the immune system begins acting with disproportionate force, leading it to harm the body. For example, the immune system may become oversensitive to foreign bodies, such as pollen, that don’t pose an actual threat, which leads to allergies. Or it might start attacking its own organs, resulting in autoimmune diseases. Even muted immune activity for long periods of time, such as low-grade inflammation, can be harmful and is thought to contribute to everything from cardiovascular disease to depression.

Historically, there are some basic ways to assess the health of your immune system – including, for instance, measuring the levels of inflammatory molecules and numbers of immune cells within your blood. But recently, our measures have become more sophisticated, with a greater focus on examining the precise balance of the immune system’s different components.

Sunil Ahuja’s team at the University of Texas Health Science Center, for example, has designed a way to classify people’s “immune grade” based on the relative levels of different T-cells. People with higher-than-average numbers of helper T-cells, combined with lower levels of killer T-cells, had the highest immune grade, according to their classification. The team has shown that people with immune grade I tend to have much better outcomes when they face a pathogen, with 88 per cent reduced mortality compared with those of lower grades after a covid-19 infection, for instance.

Importantly, the team’s grades proved to be a better predictor for response to covid-19 than age alone, meaning that, in the future, this kind of measure may help to identify high-risk individuals with greater precision. “Even if you are 80, and you have a high immune grade, you’ll do much better than someone much younger with a low immune grade,” says Ahuja.

You may need to wait a while before your doctor is able to assess your health with this kind of precision, but you can get a good idea about the state of your immune system without any tests. As a general rule of thumb, Macciochi says that the average person should expect to have one or two mild illnesses a year. If you tend to become sick far more often, and if those infections tend to last much longer and are more severe than the people around you, then it may be a sign that one or more of the units in your immune army aren’t functioning as they should. Your doctor should be your first port of call, but there are also things you can do to give your immunity a helping hand (see “The food and drink that really can boost your immune system”, above).

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