Obesity is a common condition these days, especially in Ireland, where more than a quarter of adults are considered obese. It's not just about eating too much and not moving enough. Our bodies are complex, and hormones play a big role in controlling our weight by telling us when we're hungry or full. Sometimes, things go out of whack. Stress, sedentary work, some medicines and even our genes can contribute towards weight gain by upsetting this hormonal balance.
It's not just about looking good. Excess weight can interfere with our immune system, making it easier to get sick and making illnesses hit us harder. This was very obvious during the Covid-19 pandemic. People living with obesity are more likely to have heart problems and diabetes, and now we know they're at higher risk for serious infections and some cancers too. It's all connected to how obesity can impact the immune system.
The immune system
The immune system is a complex network of cells that work in harmony to provide defence against harmful infectious agents or pathogens such as bacteria, viruses and parasites but also abnormal cells such as cancer cells.
Our body encounters pathogens on a regular basis, but our immune system keeps them at bay without us even noticing. This is because the skin and the nasal passage act as the first barriers to infections. If the pathogen does get through, there is a second line of defence called the "innate" immune system that can jump into action at the smallest sign of trouble.
The cells of the innate immune system are like soldiers patrolling your body. They have sensors that can recognise any foreign cells (infectious agents) or abnormal cells (cancer cells). When they find something suspicious, they can destroy it right away or they can send out signals, called cytokines (cyto=cell; kine=movement). These signals alert other, more specialised cells of the "adaptive" immune system, so called because the cells in this category can adapt their receptors to specifically recognise only one type of pathogen. The adaptive system include B cells which produce specific antibodies which tag a pathogen for killing and T cells which can multiply and rapidly eliminate a pathogen.
From Science ABC, how our immune system works
After an infection has gone away, it is important for the immune cells to shrink back to inactivity. Although the antibodies and some of the T cells lurk around in our blood stream so that when that pathogen is re-encountered, these cells jump into action rapidly as they "remember" it and eliminate it before it can manifest into a disease. This is what we mean by being "immune" to a certain infection.
How does obesity affect the immune system?
When we consume food, our stomach breaks it down into smaller pieces right down to the nutrients they contain. These nutrients enter into our bloodstream where they travel into different parts of the body. One of the main fuels our body uses is glucose, a fuel that keeps the engine of our body running (like petrol in our cars).
Obesity can mess with how a person's body handles energy from food, leading to what we call metabolic syndrome. This is a set of warning signs that include increased glucose in the blood stream, inability to use insulin (insulin resistance), increased amount of triglycerides (a type of fat) in the blood, and decreased levels of HDL cholesterol, which is the "good" kind of cholesterol.
From Nature, immunologist Prof Lydia Lynch on the impact of fat on the immune system
These disturbances get detected in the blood as abnormal signals that keep setting off the immune system causing what is known as chronic low grade inflammation. This background inflammation throughout our body doesn’t cause obvious problems in the short term, but can wear down the immune system over time. Like a car that is left with its engine running, the ever vigilant immune cells get exhausted. This means that they can miss signs of trouble, like an abnormal cell that could turn into cancer.
The "always-on" state of inflammation may also send the immune system into an over-drive in response to an infection causing harm to body’s own tissues. This phenomenon called the "cytokine storm" was observed widely in Covid-related deaths. My own research focus is on an immune cell type known as MAIT cells. In obesity, these cell numbers diminish and the ones that are present turn against us, disrupting insulin signalling and worsening insulin resistance.
But all is not lost. Our research has found that the drug being used to treat obesity and diabetes, GLP1, also has a positive effect on the immune system directly, improving the functionality of cancer fighting cells known as Natural Killer cells.
Our current focus is to find therapeutic targets that would stop MAIT cells turning against us in obesity and continue to remain our "MAITS". The hope is that future research will help us discover new therapies to prevent adverse outcomes and improve quality of life for people living with obesity.
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The views expressed here are those of the author and do not represent or reflect the views of RTÉ
