By Anna Maxted

Professor John Trowsdale has that (if I may) slightly wild-haired appearance that denotes a busy retirement — writing papers, music practice, minding grandchildren. An emeritus professor of immunology at the University of Cambridge, he has spent more than 50 years in research. No wonder his latest endeavour, What the Body Knows: A Guide to the New Science of Our Immune System, runs to 400 pages.

Trowsdale, 75, is speaking from his cosy, cluttered study — brass instruments freestanding, vintage film posters framed on the walls — and is affable with a laid-back air. Yet in the way of rigorous scientists, he’s exceedingly cautious about confirming anything as fact — even, on one occasion, a quote from his own book.

Thankfully, his enthusiasm gets the better of him, because he’s an entertaining educator, exhorting us to think of a rotting body to appreciate what state we’d be in without the immune system. “It’s really important for repair as well as defending against infection … it’s marvellous. You go from this wound to completely new tissue!”

He clarifieswith concision complex mechanisms, such as how some cancers are cured by harnessing the immune system: “The immune system is incredibly powerful but it is also incredibly specific.” So if you can find out what’s different about the cancer cells, “you can target just the cancer cells”. As the immune system response is finely balanced, you do this by “taking the brakes off. And T-cells — a type of white blood cell that helps the body fight infection and disease — go in and they attack the tumour.”

Trowsdale is also dubious about popular practices such as leaping into an ice bath after sport to slow inflammation, saying there’s insufficient evidence to support its effectiveness. “As usual, it may be hard to improve on nature,” he says.

Here’s what else he wants us to know …

What is the gut microbiome?

Only recently have we begun to understand the starring role our gut microbiome plays in our immunity. “Nowadays, it’s realised that the bulk of the immune system is in the gut, and we’ve really just ignored it,” Trowsdale says. A healthy microbiota protects us by regulating our immune response so it doesn’t overreact to infection and trigger chronic inflammation. So how do we assemble a solid arsenal? Infants acquire bugs from their mother’s vaginal tract during birth and through breastmilk, “shaping the composition of the microbiome”.

The range of benign bacteria we’re exposed to in the next few years is critical. This is because we’re building our unique microbiome, and for the first three years of life, the immune system is “malleable”, he says. Particularly during this time, the adaptive branch of our immune system learns from new encounters with potential threats — and is thereby “conditioned” to respond appropriately. So early exposure to beneficial germs may protect us from several chronic autoimmune conditions and allergies, influencing our health for life.

What does exposure to ‘good dirt’ mean?

“There’s probably a vulnerable period with children till about the age of three where they’re very prone to infection and the immune system is still a bit immature,” Trowsdale says. Yet a balance must be struck. “Letting kids put their fingers in their mouths and play in a bit of dirt is now permissible. Whereas before everyone was scrubbing down the surfaces and not allowing kids any exposure to any normal bacteria.”

As a father, he sterilised bottles. As a grandfather, he’ll “just wash them out”. “It’s important to keep some kind of hygiene and cleanliness but not to go overboard because of this conditioning for allergies later on in life,” he adds.

Trowsdale also cites research by the scientist Mel Greaves at the Institute of Cancer Research, on a childhood cancer: “He looked at identical twins, and one would get leukaemia, and the other, with exactly the same genetic problem, didn’t.” If the immune system was primed by exposure to other children and consequently more microbes, were they more protected? This was supported by his further findings — incidence of leukaemia was lower in children who had older siblings or went to playgroup. “He traced this to the microbiome … So the child without exposure to a greater range and diversity of bacteria, to make it simplistic, their cells tended to over-divide, and over-division then promoted this effect of inflammation, and kicked it into this cancerous condition.”

Why are allergies rising?

“Asthma and allergies, in the Sixties even, were an obscure condition,” Trowsdale says. Now, he agrees, “there’s been an explosion”.

Food allergies have doubled in the space of ten years, according to recently research published, with young children particularly affected.

Data from GP surgeries in England showed that 4 per cent of under-fives had a “probable” food allergy in 2018; in 2008 it was 1.2 per cent. Over the same decade the prevalence of food allergies across all age groups has increased from 0.4 per cent of the population to 1.1 per cent.

My family’s hypersensitivities reflect that — between five of us: penicillin allergy, Graves’ disease, asthma, eczema, hay fever, hypersensitivity to nickel, cat fur, essential oils. “Wow,” he says. “That’s a lot.” Why has it kicked off? Potential contributors include “use and misuse of antibiotics, a high-fat high-sugar diet, birth by caesarean section, the urban environment, pollution, childhood infection and feeding babies formula milk”. Notably, a common factor is that all impact the microbiome.

Your heart does sink at the milk news — the breast v formula debate is so toxic. However, while Trowsdale says, “We know now that some of the antibodies you get from breastmilk you can’t manufacture,” he’d hate to “frighten” mothers. “Plenty of people were bottle-fed and don’t have any allergies,” he stresses. Other allergy risk-reducing strategies: ensure children get enough vitamin and, if you have a dog or cat when your kids are small, marvellous. They’re a vector to the outdoors, and studies suggest the “mini-farm” effect protects against allergy development. Although for those who already have a hypersensitivity, let’s hope you didn’t buy that labradoodle under false pretences. Trowsdale notes: “There’s no such thing as a hypoallergenic dog or cat.”

Dry skin is a risk factor

Why would avoiding dry skin reduce allergy risk? Trowsdale says: “Young children may be sensitised to allergens by exposure through the skin, but tolerised to them in the gut.” That is, eating a “foreign protein” like peanut butter doesn’t provoke an allergic reaction — but it does if the first encounter is absorption through a break in the skin for example, if a baby has eczema.

So if children have had a peanut-free diet but are “exposed to it later through the skin, through kissing, through contact with your mother who’s eaten a peanut butter sandwich, or through peanut dust even in an aeroplane — the immune system interprets that as a kind of threat. And then it sets up this allergic reaction.” It’s now recommended giving infants peanut butter (not peanuts as they might choke) soon after weaning, “because that then conditions the immune system”.

Can you ‘boost’ immunity?

Diet in early life has an impact, Trowsdale says. “We’ve got to prime our microbiome by eating a variety of foods the so-called ancient ancestors used to eat — nuts, berries, vegetables, fruits and fibre — so there’s enough different species in there that they help to condition the immune system.”

Can we “boost” our immunity throughout life this way? He thinks the word is overused — even vitamin C “doesn’t prevent colds”. But we can look after our immunity by eating well. In addition to Stone Age-adjacent fare, he’s a fan of kefir and sauerkraut (“fermented things”). Consider the immune system a Rolls-Royce, he suggests: “You can destroy it if you put water in the tank.” He says, “if you’re smoking a lot, drinking too much, not exercising, it has a detrimental effect. The immune system is not easy to boost, it’s easier to damage.”

Why does the World Health Organisation class obesity as a ‘disease’?

It has previously been thought “that having ample fat stores was desirable, potentially as a buffer against illness”, Trowsdale says. “Now it’s a disease.” He defines “disease” as “damage to tissues”: “when you overeat or get too fat, that tissue is damaged”.

Essentially, overloading fat cells stresses the tissues containing them, and immune cells think, “There’s damage, I’m needed!” They rush in, creating low-level persistent inflammation, “so-called sterile inflammation”, Trowsdale says. “You’re getting an inflammatory reaction and the immune system is being activated but there’s no infection, you’ve just got too much fat.”

Gut bugs play a role: “Microbiome is thought to be a big factor in pushing us towards obesity.” As for ultra-processed foods, they’re “evolutionarily akin to a new virus”. If you’re eating a high-fat, ultra-processed diet, you encourage pro-inflammatory microbes to flourish. He says: “There’s a knock-on effect of the microbiome on obesity, on eating more, and also on propensity to put on fat.” He notes that in twin studies by the epidemiologist Professor Tim Spector, Akkermansia and Christensenella (microbes, not top west London baby names) were more prevalent in the “thinner” twin. Both are available as probiotics.

Anti-ageing the immune system

The immune system deteriorates with age. We aren’t producing any new T-cells, which play a critical role in immunity. And “many of us are carrying around persistent viruses” — Epstein-Barr virus, for instance — and if a tenth of our old but hopefully wise T-cells are busy combating these viruses, we have less capacity to fight other infections.

But we’re not powerless. Trowsdale cites the immunologist Professor Janet Mary Lord’s research: “You can actually delay the immune system’s deterioration and increase longevity by moderate exercise. She believes that promotes anti-inflammatory factors.” Cycling, for instance, and “fasting seems to have a similar effect”, he adds. Or calorie restriction: after Yale researchers recruited volunteers to eat 14 per cent fewer calories for two years, they had better T-cell production, ie “more robust adaptive immunity”.

Why stress can make you more susceptible to illness

On how the “constant drizzle” of stress can affect immunity, Trowsdale explains that consistently higher levels of cortisol lead to chronic “non-specific inflammation” — in other words, “an immune system that can’t switch off”. It can mean that we’re more susceptible to infection because, broadly speaking, our immune cells are “exhausted”. He adds: “We’ve got two things in our body that use a lot of energy, the brain and the immune system. When you’re stressed and the brain is working full-time, possibly some of the resources to the immune system are not as powerful as they should be.”

And the kicker — sleep deprivation, so often a consequence of stress, also increases inflammation. How to calm ourselves? Yoga or slow breathing could help. Trowsdale says: “Long diaphragmatic breaths … encourage the vagus nerve to tell the brain to relax the body and potentially control inflammation.” We tend to think of the brain as giving the orders, he notes. “There’s an awful lot of body to brain going on.”