The Body’s Fight Against Respiratory Viral Infections

Written by: Omar ElDimassi

Edited by: Yasuko Yoshida, Ariel Yu, Kristine Chung, Emily Wang, Kayla Joseph

Illustrated by: Gabe Cruz

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Introduction

Have you ever asked yourself why we get the cold every winter season? Respiratory viral infections are among the most common illnesses affecting humans, ranging from mild colds to severe diseases such as influenza and COVID-19. These infections primarily target the respiratory tract, the nose, throat, and lungs, and are typically spread when a person inhales virus-containing moisture released into the air by coughing, sneezing, or talking. Once inside the body, viruses invade host cells and hijack their machinery to reproduce, triggering the body’s complex immune defense systems.

The immune system operates through two primary components: innate immunity and adaptive immunity. The innate immune system serves as the body’s first line of defense, rapid but nonspecific, while the adaptive immune system is slower but is a highly specialized attack that can remember and respond more effectively to the same virus in the future.

However, not all immune responses are equal. A person’s lifestyle, including diet, sleep, and stress management, significantly influences immune function. Nutrient-dense foods and natural immune enhancers can strengthen cellular defenses, while poor nutrition and chronic stress can weaken them. Understanding these biological and environmental interactions provides insight into why some individuals recover quickly from infections while others develop severe symptoms.

Lifestyle Differences and Immune Enhancement

The immune system thrives on a balanced internal environment. Certain foods, known as immune enhancers, play a vital role in supporting this balance. These can be natural food sources, such as fruits rich in vitamin C,  oily fish containing omega-3 fatty acids, or supplements that supply essential micronutrients like zinc, vitamin D, and selenium. Vitamin C supports the function of neutrophils, a type of white blood cell that engulfs pathogens and enhances epithelial barrier function (acting as a sort of gate keeping that traps germs and dust from the outside world) [1]. Vitamin D regulates antimicrobial peptides (small proteins that act as defense mechanisms against germs like bacteria) in the respiratory tract and modulates T-cell responses, reducing the risk of severe viral inflammation [2]. Zinc helps with the development of immune cells and directly inhibits viral replication and development [3].

Lifestyle habits such as adequate sleep, hydration, and regular physical activity also modulate immune responses. Sleep deprivation and high cortisol levels can lead to body inflammation by increasing inflammatory molecules [4]. Such internal inflammation has proven to stop lymphocyte production, thus weakening adaptive immunity [2]. Thus, diet and daily habits are factors that influence how resilient one’s immune system is against respiratory viruses [5].

The Body’s Immune Response

When a respiratory virus enters the body, through inhalation, the innate immune system immediately responds [6]. The epithelial cells of the respiratory tract act as physical and chemical barriers, producing mucus that traps pathogens. Cilia, small hair-like structures lining the airways, sweep out these trapped particles.

If the virus is still able to pass those barriers, macrophages and dendritic cells detect general molecular patterns common to pathogens through their pattern recognition receptors. These immune cells release cytokines, which are chemical messengers that recruit other immune cells and create inflammation to contain the infection [6].

Next, the adaptive immune system takes over. B cells create antibodies that recognize the virus and help stop it. Helper T cells guide the rest of the immune response, and cytotoxic T cells destroy infected cells by causing them to self-destruct [7].

The Immune System Without External Support

Even in the absence of external enhancers or medications, the immune system has intrinsic mechanisms to fight off respiratory viruses. Upon infection, innate immune responses quickly produce interferons (IFNs), which are antiviral proteins that limit viral replication and activate natural killer (NK) cells [6]. NK cells destroy infected host cells before the adaptive immune system is fully mobilized [8].

After several days, adaptive immunity then takes control. B cells differentiate into plasma cells that release virus-specific antibodies, while memory cells work through long-term immunity. For example, after recovering from influenza or COVID-19, the body retains immunological memory that can prevent reinfection or lessen severity [8].

However, the strength and duration of this memory depend on the virus. The common cold, caused by over 200 different viral strains, remains difficult for the immune system to recognize universally. Therefore, people often get multiple colds each year [8].

Contemporary Views and Advances

Recent research has deepened our understanding of how different viruses interact with the immune system. For example, influenza viruses frequently mutate their surface proteins, evading existing antibodies and requiring annual vaccines [8]. SARS-CoV-2, the virus responsible for COVID-19, uses the specific receptors on lung and nasal cells to gain entry, which has prompted the development of mRNA vaccines that train the immune system to recognize its spike protein and be able to defend when another viral of COVID-19 happens again [8].

It is much easier to create an engineered defense mechanism like the mRNA for COVID-19, as the virus being used to infect the body cells are unique and known to scientists. Those viruses have been included into our medical data and we use them as reference for medications. The common cold, by contrast, remains an unsolved biological problem. Because so many viral species cause it, there is no single “cold virus” to target, thus the mRNA concept could work in theory but only if we can classify each common cold infection and be able to know which exact virus is infecting the body, or create an mRNA that could potentially, with future advances, target many viruses at once [5]. 

Conclusion

The body is challenged every year in multiple ways through many respiratory viral infections, but the immune system is equipped with layered defenses that work together to protect us. From rapid, nonspecific actions of the innate immune system to the targeted, longer lasting protection of adaptive immunity, the body uses strategies to identify and eliminate those viral threats. Maintaining balanced lifestyle habits such as nutrition, sleep, and stress management also play a meaningful role in shaping how effectively these defenses operate, explaining why responses to the same infection can vary widely among individuals. Even with scientific advances, such as mRNA vaccines, have strengthened our ability to prevent and treat these illnesses, the immune system itself remains our most powerful tool. Understanding how it functions, and how we can support it through healthy habits, helps show us how we fight respiratory viruses and how we can better protect our health in everyday life.

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References

[1] Lee WJ. Vitamin C in Human Health and Disease: Effects, Mechanisms of Action, and New Guidance on Intake. Springer; 2019.

[2] Moro-García MA, et al. Influence of inflammation in the process of T lymphocyte differentiation: proliferative, metabolic, and oxidative changes. Front Immunol. 2018;9. Published March 1, 2018.

[3] Wessels I, Maywald M, Rink L. Zinc as a gatekeeper of immune function. Nutrients. 2017;9(12). Published November 25, 2017.

[4] Harvard Health Publishing. How sleep deprivation can cause inflammation. Published April 29, 2024. 

[5] Ricci A, Roviello GN. Exploring the protective effect of food drugs against viral diseases: interaction of functional food ingredients and SARS-CoV-2, influenza virus, and HSV. Nutrients. 2023;15. Published February 1, 2023.

[6] Burrell CJ, Howard CR, Murphy FA. Innate immunity. In: Fenner and White’s Medical Virology. Elsevier; 2016.

[7] Markman M. B-cells vs. T-cells: what are helper, killer, and cytotoxic T cells? City of Hope. Published July 12, 2022. 

[8] Gambadauro A, et al. Immune response to respiratory viral infections. Viruses. 2024;16. Published June 4, 2024.