The Science of Immunity: Why Do Some People Get Sick More Often?

13 May 2025

8 Min Read

Dr Ooi Yin Yin (Academic Contributor)

IN THIS ARTICLE

Contributed by Dr Ooi Yin Yin, whose research focuses on neuroinflammation and cellular immunology. She can be reached at yinyin.ooi@taylors.edu.my.

You and your friend live in the same dorm, attend the same classes, and are exposed to the same environment—yet while you barely get sick, your friend seems to catch every cold going around. Why does this happen? Is it just bad luck, or is there something more at play?

What Is Immunity?

Immunity refers to the body’s ability to resist and fight off infections, such as those caused by bacteria, viruses, and other harmful microorganisms. Think of it as your body’s personal army, always on alert to identify and neutralise threats. This system is incredibly sophisticated and consists of two main branches: innate and adaptive immunity.

Innate immunity is the body’s first line of defence. It includes physical barriers like the skin and mucus, along with general-purpose immune cells that launch quick, non-specific attacks against invaders. It’s fast, but not very precise. Adaptive immunity, on the other hand, is more like a specialised intelligence unit. It involves T-cells and B-cells that learn from past infections and 'remember' how to respond more effectively the next time the same pathogen appears.

The Genetics of Immunity: Born Strong or Vulnerable?

While we all have immune systems, not all are created equal. Genetics play a key role in determining how well your body can defend itself. Some people are born with genes that make their immune system more efficient, while others may inherit genetic traits that increase their susceptibility to illness or even lead to autoimmune disorders.

One of the key players in your immune system’s defence strategy is a group of genes known as Human Leukocyte Antigens (HLA). Think of HLA molecules as the barcode presenters of your body—they display tiny fragments of proteins (including those from viruses and bacteria) on the surface of cells so that T-cells, the immune system’s ‘ID-checkers,’ can inspect them. Some people have HLA types that do an excellent job at flagging dangerous intruders, offering stronger protection against diseases like malaria or hepatitis. But in others, these same genes can misfire—presenting the body’s own cells as threats. This can confuse the immune system, leading to autoimmunity, where it mistakenly attacks healthy tissue.

_{Your immune system is supposed to protect you—but in rare cases, it can mistake your own brain for a threat. In a condition called autoimmune encephalitis, the body produces antibodies that attack the brain—causing hallucinations, memory loss, and even sudden personality changes.}

Here’s a fascinating fact: some people are naturally resistant to Human Immunodeficiency Virus (HIV)—all because of a rare genetic mutation called CCR5-Δ32. This mutation acts like a locked door, stopping the virus from entering and infecting immune cells. But HIV isn’t the only illness affected by our genes. The production of cytokines—tiny chemical messengers that help coordinate immune responses—are also influenced by our DNA.

Hospital COVID Healthcare workers during an intubation procedure to a COVID patient

_{Some people have gene variants that cause their bodies to overreact to infections, triggering excessive inflammation. During the COVID-19 pandemic, this overreaction turned what could have been a mild case into a serious, even life-threatening condition for some individuals.}

How Your Daily Habits Shape Your Immunity

You might not be able to change your genes, but your daily choices can either strengthen or weaken your immune system. Your everyday habits—sleep, exercise, diet, and stress—play a big role in how well your body fights off illness.

Let’s start with sleep—your immune system’s secret weapon. When you sleep deeply, your body gets to work producing cytokines and infection-fighting cells. Skimp on sleep, and your defences take a hit. One study published in Sleep found that people who clocked less than six hours of sleep a night were four times more likely to catch a cold than those who got more than seven hours.

_{Sleep isn’t just rest—it’s immune system maintenance. T-cells become less efficient, while natural killer (NK) cells decrease in number and activity when you’re sleep-deprived, reducing your body’s ability to detect and destroy infected or abnormal cells.}

Then there’s stress. A little stress before a big presentation might sharpen your focus, but chronic stress is a different story. When stress sticks around, it raises levels of cortisol, a hormone that suppresses your immune system and ramps up inflammation. Over time, this weakens your body’s ability to defend itself, making you more prone to getting sick—and staying sick longer.

Exercise, when done right, is like giving your immune system a tune-up. Just 30 to 60 minutes of moderate activity a few times a week boosts blood flow, helping immune cells move around the body more efficiently. But here’s the catch: too much intense training without rest can actually backfire. Overtraining puts your body under constant stress, raising cortisol and increasing inflammation, which eventually wears your immunity down.

Diet and Gut Health: The Fuel for a Strong Immune System

Your immune system is only as strong as what you feed it. The nutrients you consume are the building blocks your body needs to produce immune cells, send the right signals, and mount a defence when viruses or bacteria strike.

Certain nutrients are especially important:

Vitamin C, found in fruits like oranges and vegetables like bell peppers, helps boost the production and function of white blood cells—your immune system’s foot soldiers.
Vitamin D, which your body makes when exposed to sunlight and which is also found in fatty fish like salmon, plays a critical role in activating immune defences.
Zinc, present in foods such as meat, nuts, and seeds, supports immune cell communication and helps coordinate your body’s response to infection.

Staying hydrated is just as important. Water helps transport immune cells through the bloodstream and supports the removal of waste products and toxins that can compromise health.

Microbiome intestine factories and microbiota

_{But immunity isn’t just about vitamins—it also lives in your gut. Roughly 70% of your immune cells are based in your digestive tract, where they interact constantly with your microbiome—a vast ecosystem of bacteria that influence everything from digestion to immune training. A balanced microbiome affects your immune system to reducing the risk of overreaction or missed threats.}

Incorporating probiotic-rich foods like yoghurt, kimchi, tempeh, and other fermented products can help maintain a healthy microbial balance. One study published in Nutrients found that university students who consumed probiotics regularly had fewer sick days and reported feeling healthier overall.

Simply put, what you eat doesn’t just fuel your body—it programmes your immune system, too.

Biotech and Immunity: Science That’s Rewriting the Rules

We’re no longer just learning about the immune system—we’re starting to upgrade it. Thanks to breakthroughs in biotechnology, scientists are finding new ways to supercharge our body’s defences, turning the science of immunity into personalised, cutting-edge medicine.

Reprogramming T-Cells with CRISPR

Imagine being able to edit your immune cells like you edit a document. That’s what CRISPR gene editing is now making possible. Researchers are using this powerful tool to tweak T-cells—your immune system’s elite fighters—so they can hunt down and destroy cancer cells more effectively.

_{One of the most exciting developments is in CAR-T therapy, where T-cells are reprogrammed to recognise specific cancer targets. A 2024 NIH-linked study reported that these modified cells are showing real promise, even against cancers that were once considered untreatable.}

The Rise of AI in Immune Medicine

What if your doctor could predict how your immune system would respond to a specific treatment—before you even tried it? That’s where artificial intelligence (AI) comes in. By analysing your genetic data, lifestyle, and even gut bacteria, AI tools can now help doctors create personalised vaccines and immune therapies. Think of it as precision medicine: the right treatment, for the right person, at the right time.

Factors like epigenetic changes, reduced microbial exposure, latent infections, and the virome can profoundly influence our immune responses and long-term immune health too.

— Dr Ooi Yin Yin

Conclusion

Your immune system isn’t fixed—it’s something you can shape every day. While genetics play a role, your choices around sleep, food, stress, and movement make a real difference. With every smart habit, you’re not just preventing illness—you’re building resilience.

In understanding how your body defends itself, you’re also learning how to take charge of your health. And that’s powerful.

Ready to explore the science behind the human body—and shape the future of health? Begin with the Foundation in Science and advance into Biomedical Science, here discovery becomes impact.

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