What is the immune system, and how does it work? Organs, cells, and immune responses explained

What is the immune system?

The immune system is your body’s built-in defense network. It helps identify and respond to foreign challenges to support normal functions and processes in the body.
The immune system is not a single organ; it’s a network of internal parts that either contain immune cells, produce them, or help them do their jobs. White blood cells are the main actors. They exist in the bloodstream and lymphatic system, working in organs like the lymph nodes, spleen, bone marrow, and thymus.

Key functions of the immune system

The immune system identifies changes in the body’s environment, creates a response, remembers past exposures, and supports normal body function. More specifically:

• It helps identify potential challenges to the body—such as foreign challenges and environmental stressors—while recognizing the body’s own cells.
• It responds to these challenges and supports the body’s natural defense processes. Some immune defenses respond quickly and broadly, while others provide targeted support using specialized cells and antibodies. The system works to support the body’s natural processes, helping maintain cellular health and supporting tissue function.
• It creates defense memory. After encountering a challenge, either through exposure or prior encounter with a challenge, the immune system remembers the specific challenge, allowing it to respond faster and more effectively the next time the body is exposed to that challenge.^1,2

What are T Cells and their role?

We can’t talk about the immune system without mentioning T Cells. These are a type of white blood cell that plays a central role in the immune system. T cells are made in the bone marrow, but they mature in the thymus—a small organ in your chest, just behind the breastbone. That’s what the “T” stands for.
T cells are immune-specific and help identify stressors or unfamiliar cells, coordinate immune responses, support the body’s natural cellular processes, and remember specific challenges so the body can respond more efficiently over time. That memory is part of what’s called the adaptive immune system, which learns and adapts according to previously confronted challenges and helps support immune balance.³

What factors influence immune function?

Immune function naturally varies based on many factors. When the body lacks adequate nutrients or experiences stress, immune responses may be impacted. Nutrition, lifestyle, stress levels, and sleep quality all play a role—and their effects can be short-term or ongoing.
Adequate nutrition is important for supporting normal immune function. Common nutrient gaps, digestive challenges, or dietary imbalances can affect how well the body maintains its immune defenses. Other lifestyle factors, such as stress, poor sleep, smoking, heavy alcohol use, and inactivity, can further impact immune responses. Nutritional needs may also change with age.⁴
Getting the necessary amount of vitamins, minerals, antioxidants, and other health-supporting nutrients from a diet rich in whole foods—every day—can help support normal immune function.

Supporting your immune system

The immune system works best when your core health needs are met consistently.

Foundational health matters: sleep, nutrition, stress management, and exercise.

• Immune cells regenerate during sleep, so sleep loss can affect immune responses.⁵
• Eating enough calories, protein, and a variety of fruits and vegetables provides the nutritional resources immune cells need to function normally.
• Stress releases hormones that can impact immune health.⁶
• The immune system is also very responsive to exercise: regular, moderate movement supports immune health by improving natural immune function and decreasing stressors.⁷

A strong immune function is well supported by consistent health habits. Get adequate nutrition, move your body regularly, keep your stress in check, and avoid behaviors that can chronically stress your body.

Common myths about the immune system

There are many widely accepted ideas about the immune system that are not correct. Let’s clarify some of those.

Myth: Supplements can replace healthy habits.

Fact: Vitamins and supplements are an important tool for refilling nutrient gaps, and they work best alongside a healthy diet. Only about 10–12% of US adults eat the recommended amounts of fruits and vegetables, which can lead to nutrient gaps.⁸ Providing your body with sufficient nutrition contributes to overall immune health.

Myth: If you experience immune challenges often, it’s somehow your fault.

Fact: Your immune system responds to your environment. Kids in classrooms, teachers, healthcare workers, and anyone who frequently engages with lots of other people are inevitably exposed to more immune challenges throughout the day. In this circumstance, and as always, following healthy habits—diet, nutritional support from supplements, exercise, and good sleep—can support immune health and immune defenses.

Myth: Cold weather directly impacts immune function.

Fact: Temperature itself doesn’t suppress immune response—the environmental factors that change in winter are what matter.⁹ In winter, people spend more time indoors in closer quarters with less ventilation, which increases exposure to environmental challenges that can affect normal immune function.

Myth: A sterile environment is best for health.

Fact: Normal environmental exposure, especially during development, supports the immune system’s natural maturation process and natural immune function.¹⁰ It’s actually good to be exposed to various microbes—for your body to learn how to respond to them.¹¹ Humans have co-evolved with microbes over millennia, and we depend on them for healthy immune function. The age-old method for such exposure? Get out in nature and play in the dirt!
Understanding how your immune system really works and how to support it is a major part of tending to your long-term health. Prioritizing foundational nutrition is essential. Sleep, stress management, and exercise matter too—and getting the necessary vitamins, minerals, and nutrients will contribute to the ongoing well-being of your body’s immune health.

References:

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8. Passarelli S, Free CM, Shepon A, Beal T, Batis C, Golden CD. Global estimation of dietary micronutrient inadequacies: a modelling analysis. Lancet Glob Health. 2024; published online Aug 29. doi:10.1016/S2214-109X(24)00276-6
9. Castellani JW, Brenner IK, Rhind SG. Cold exposure: human immune responses and intracellular cytokine expression. Med Sci Sports Exerc. 2002;34(12):2013-2020. doi:10.1097/00005768-200212000-00023
10. Umetsu DT. Early exposure to germs and the Hygiene Hypothesis. Cell Res. 2012;22(8):1210-1211. doi:10.1038/cr.2012.65
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