Lymphopenia, also known as lymphocytopenia, refers to a condition where there is a low level of lymphocytes in the blood. Lymphocytes are a type of white blood cell, and they are crucial to the body’s immune defence, particularly against viruses. Whilst lymphopenia can occur as a result of a variety of health conditions, viral infections are one of the most commonly associated conditions. Viruses such as HIV, SARS-CoV-2 (COVID-19), influenza and hepatitis can lead to lower lymphocyte levels and ultimately impact the body’s immune system.

Read on to learn more about lymphopenia in viral infections, the underlying mechanisms and the clinical impact of lymphopenia. 

Understanding Lymphocytes & Lymphopenia

Lymphocytes are a type of white blood cell and are one of the body’s main immune defenders. They include T lymphocytes (T cells), B lymphocytes (B cells) and natural killer (NK) cells, with each type playing a distinctive role.¹ T and B cells are part of the body’s adaptive immune system, whilst natural killer cells are part of the body’s innate immune system. The innate immune system, which NK cells are a part of, is a general and non-specific immune system which is the body’s first line of defence. The adaptive immune system, on the other hand, is a specialised immune system which works by targeting specific pathogens and creating a long-term immune memory which allows the body to fight off these specific pathogens in the future.² Because of the differences in origin, function, and response speed, NK cells can sometimes be classified separately from T cells and B cells, which are adaptive lymphocytes. However, all three are crucial to boosting the immune system, and they can all be affected by lymphopenia. 

Lymphopenia occurs when the level of lymphocytes drops below the normal range, which is typically below 1000 cells per microlitre in adults and older children. Whilst viral infections are one cause of lymphopenia, it can be caused by various other conditions, such as:³

• Bacterial infections
• Cancer
• Chemotherapy
• Sepsis
• Auto-immune diseases 
• Genetic factors

Lymphopenia can range from mild to severe and can often be a manifestation of immune dysfunction or dysregulation. Lower lymphocyte levels can have an impact on the body’s immune defence and weaken the immune system. This makes it harder for the body to fight off pathogens and can make the body more susceptible to infections. Lymphopenia is a common characteristic of viral infections and can be a useful feature when considering disease progression and clinical course.  

Underlying Mechanisms of Lymphopenia in Viral Infections 

Lymphopenia from viral infections is not a result of a singular process, but instead, a combination of factors. These underlying mechanisms range from inducing lymphocytic cell death to disrupting the production of lymphocytes. Understanding these underlying mechanisms can allow us to gain a clearer picture of why some viral infections lead to severe complications and prolonged disruption of the immune system. 

Viral infections can trigger lymphopenia through various mechanisms, which include: 

Viral Cytopathic Effects: Viruses Can Directly Destroy Lymphocytes

Some viruses have the ability to directly infect and destroy lymphocytes. This is known as a “cytopathic effect”, which refers to destructive or harmful changes caused by a virus, and typically involves structural and pathological changes to the cell.⁴ 

Essentially, this is how it works in regards to lymphocytes:

• A cytopathic virus can enter a lymphocyte and use it to replicate
• As the virus replicates and multiplies, it can then produce toxic viral products or destructive enzymes. This could then cause a disruption in the body’s normal cellular processes
• As a result, the lymphocyte infected by the cytopathic virus can be damaged and often die⁵ 

One of the most prominent examples of the cytopathic effects of a virus is with Human Immunodeficiency Virus (HIV), where the HIV virus kills specific lymphocytes known as CD4 T cells.⁶ Additionally, some research has also shown that SARS-CoV-2 could potentially have cytopathic effects by infecting and killing T lymphocytes.^7,8 Ultimately, these cytopathic effects could lead to lower levels of lymphocytes in the blood. 

Lymphocyte Apoptosis: The Body’s Immune Response Can Trigger Cell Death 

Even in viruses that don’t necessarily have cytopathic effects and kill lymphocytes directly, viral infections can still cause an immune response that triggers cell death, known as apoptosis, in lymphocytes. Whilst apoptosis is a normal cellular process that happens in the body, viral infections can cause this process to take place excessively. When apoptosis occurs excessively, it can cause a significant drop in lymphocytes. Bystander lymphocyte cells, which are cells that are not directly infected by the virus, can also contribute to lymphocyte cell death. One reason for this is the release of cytokines, which are signalling proteins that regulate the inflammatory processes in the body. Research has shown that viral infections can promote an overproduction of proinflammatory cytokines, such as IL-6 and TNF-ɑ, which can trigger lymphocyte cell death.³ Essentially, the inflammatory environment triggered by a viral infection can lead to lymphocyte cell death, ultimately contributing to lymphopenia and weakening the body’s immune system. 

Inhibition of Lymphopoiesis: Viruses Can Inhibit the Production of New Lymphocytes

Another significant mechanism underlying lymphopoiesis in viral infections is the inhibition of lymphopoiesis, which refers to the production of lymphocytes. Lymphocytes originate in the bone marrow, and some viral infections can infect the stem cells in the bone marrow that would normally produce these lymphocytes. In addition, some viruses that trigger a proinflammatory response (e.g through the release of cytokines), can create a hostile environment which inhibits or slows down the production of new lymphocytes.³ Thus, when there is a viral infection, the body can struggle to replace the lymphocytes that are lost from the infection, which leads to lower lymphocyte levels and lymphopenia. 

Clinical Impact and Diagnostic Implications

Lymphopenia can be a significant clinical marker for viral infections, particularly due to the fact that the presence of lymphopenia is associated with poorer outcomes and disease severity. For example, studies have found that individuals with an acute COVID-19 infection, as well as lymphopenia, have an increased risk of developing severe COVID-19 and tend to have poorer outcomes (e.g increased mortality).⁹ Some studies have found that the presence of COVID-19 in individuals who have lower lymphocyte levels was associated with higher rates of admission into the intensive care unit, an increased risk of respiratory distress, and death.¹⁰ In addition, lymphopenia weakens the body’s immune system, which makes it more susceptible to opportunistic infections, which are infections that occur and thrive when the body’s defence system is weakened. This is the case for viral infections like HIV, which are prone to opportunistic infections. In HIV, recurrent opportunistic infections are associated with the progression and severity of the disease.³ Moreover, lymphopenia in viral infections clinically reflects poor outcomes and a higher risk for disease severity. 

Moreover, monitoring lymphocyte levels and checking for lymphopenia can be used as a diagnostic tool for infections and can be highly useful for assessing prognosis and disease severity. 

Examples of Viral Infections Associated with Lymphopenia 

Human Immunodeficiency Virus (HIV)

HIV is one of the most well-known viral infections associated with lymphopenia. The HIV virus infects and destroys lymphocytes, particularly CD4 T lymphocytes, which contributes to a lower level of lymphocytes in the blood. In HIV, lymphocyte count and the presence of lymphopenia can be used as a marker for predicting disease prognosis and measuring the progression of HIV.¹¹  

SARS-Cov-2 (Covid-19)

SARS-CoV-2 is another viral infection associated with lymphopenia. Some research has found that the lower lymphocyte levels in COVID-19 may be a result of an overproduction of proinflammatory cytokines as well as the promotion of T lymphocyte death, or apoptosis. Lymphopenia-associated COVID-19 is linked to increased disease severity, higher risk of mortality, and poorer outcomes overall.¹² 

Influenza (Flu)

In influenza, it is common for short-term lymphopenia to occur, and a low lymphocyte count is often used as a diagnostic indicator for the flu. Additionally, measuring lymphocyte levels can be a useful indicator of illness severity.  

Other Viral Infections

Other viral infections that are associated with lymphopenia include viral hepatitis, measles, dengue virus, Ebola virus, and viral hemorrhagic fevers. 

Summary

• Lymphopenia refers to a lower-than-normal level of lymphocytes in the blood and is a key and significant feature of many viral infections
• Viral infections such as HIV, influenza, and COVID-19 can cause lymphopenia
• There are a variety of mechanisms underlying a reduction of lymphocytes in viral infections, which include inducing lymphocyte death and inhibiting the production of lymphocytes
• Lymphopenia in viral infections can often be a useful measure of disease severity, progression, and outcome. In some viruses, such as HIV and COVID-19, the presence of lymphopenia is associated with increased illness severity and poorer outcomes
• Ultimately, lymphopenia is a significant clinical marker of immune health and can indicate the severity of viral infections, influence patient outcomes, and be used to guide clinical decisions