Introduction

In recent years, a new type of cancer treatment called immunotherapy has brought hope to many patients. Unlike traditional chemotherapy, which directly attacks cancer cells, immunotherapy works by activating the body’s immune system to fight cancer more effectively. One of the most powerful forms of immunotherapy involves Immune Checkpoint Inhibitors (ICIs). ICIs are drugs that “take the brakes off” immune cells, helping them recognise and destroy proliferating cancer cells.

While this approach can be life-saving, it doesn’t come without risks. Sometimes, the immune system becomes overactive and starts attacking healthy tissues. Such reactions can lead to a group of side effects known as immune-related toxicities. One of the early warning signs of these side effects is lymphopenia, a condition characterised by an abnormally low number of lymphocytes (e.g., a type of white blood cell) in the blood.

This article will explain what lymphopenia is, why it matters during cancer treatment, and how it can act as an indicator of potential complications in patients receiving ICIs. Understanding this connection can help patients and caregivers stay informed, monitor their health more closely, and work with their doctors to ensure safer treatment.

What are immune checkpoint inhibitors?

Our immune system is designed to detect and destroy harmful pathogens, such as viruses, bacteria, and abnormal cells that proliferate abnormally, including those associated with cancer. However, cancer can trick the immune system into ignoring it. To understand how, it helps to know that our immune system has built-in “checkpoints” molecules that act like brakes, preventing it from attacking healthy tissues by mistake.³

Immune checkpoint inhibitors (ICIs) are a type of cancer immunotherapy that releases these brakes. They work by blocking specific checkpoint proteins on immune cells or cancer cells, allowing the immune system to recognise and attack tumours more effectively.

Some of the most commonly used ICIs target:

• Programmed Death-1 (PD-1)
• Programmed Death-Ligand 1 (PD-L1)
• Cytotoxic T-Lymphocyte Associated Protein-4 (CTLA-4)

These drugs have been FDA-approved for treating several cancers, including:

• Lung cancer
• Melanoma (skin cancer)
• Kidney cancer
• Bladder cancer
• Head and neck cancers
• Some types of colorectal cancer

Familiar brand names include Keytruda (pembrolizumab), Opdivo (nivolumab), and Yervoy (ipilimumab).

ICIs have completely changed the way that cancer is treated, particularly for patients who do not react well to radiation or chemotherapy. They give people hope for a longer lifespan and, occasionally, even remission. They do, however, carry special risks, such as the potential for immune-related toxicity, a side effect in which the body attacks healthy organs.²

This is where monitoring the immune system can play a vital role in keeping treatment both effective and safe.

What is lymphopenia?

Lymphopenia, also called lymphocytopenia, is a medical term that means having too few lymphocytes in your blood. Lymphocytes are a type of white blood cell that plays a critical role in protecting your body from infections, viruses, and even cancer.

There are three main types of lymphocytes:

• B cells: produce antibodies
• T cells: help kill infected or abnormal cells
• Natural Killer (NK) cells: directly attack harmful invaders

When your lymphocyte count drops too low, your immune system becomes weaker, making you more vulnerable to infections and potentially affecting how your body responds to treatments like cancer immunotherapy.

What causes lymphopenia?

Lymphopenia can be caused by many conditions and treatments, including:

• Cancer treatments: chemotherapy, radiation, or immunotherapy
• Autoimmune diseases: lupus or rheumatoid arthritis
• Viral infections: HIV or severe cases of COVID-19
• Severe malnutrition or long-term stress
• Certain genetic or inherited conditions

In patients receiving ICIs, lymphopenia may occur when the immune system is overstimulated or damaged as a side effect of treatment. Studies have found that patients who develop lymphopenia during ICI therapy may be more likely to experience severe immune-related side effects and may have worse treatment outcomes.

How is it diagnosed?

Lymphopenia is diagnosed through a simple complete blood count (CBC) test. A normal lymphocyte count for most adults ranges between 1,000 and 4,800 cells per microliter of blood. When the count drops below 1,000, it is typically considered lymphopenia.

Your doctor may monitor this number regularly, during cancer treatment, to check how your immune system is responding to ICIs and to catch early signs of toxicity.

Why does it matter?

In the context of cancer treatment, especially with modern therapies like ICIs, lymphopenia is more than just a number. It may serve as an early warning sign that the immune system is under too much stress or that a serious side effect could be developing.

By monitoring lymphocyte levels, healthcare providers can:

• Adjust treatment if needed
• Prevent more serious complications
• Improve patient safety and outcomes

Why does lymphopenia happen during cancer treatment?

One of the most common effects seen during cancer treatment is lymphopenia, or a drop in lymphocytes, the white blood cells that help fight infections and control inflammation. This may occur for several reasons, including:

1. Chemotherapy and radiation, which directly kill immune cells

Chemotherapy and radiation therapy are effective treatments that target and destroy rapidly dividing cancer cells. However, beyond killing proliferating tumour cells, they can also hurt healthy cells that divide quickly. This may include those in the bone marrow, where blood cells – including lymphocytes – are made. As a result, blood lymphocyte count may decrease significantly during treatment.

2. Immune checkpoint inhibitors can overactivate and exhaust immune cells

ICIs, on the other hand, help the immune system fight cancer by boosting it. However, in some cases, this activity can make the immune system work too hard. Over time, the number of lymphocytes may go down because they are overstimulated, hurt, or worn out. In some patients, this can make the body more vulnerable to infections and autoimmune-like reactions.

 3. Cancer itself can disrupt the immune system

Some cancers, especially advanced or blood-related cancers (e.g., lymphoma or leukaemia), can directly affect the production or survival of lymphocytes. Tumours may release substances that suppress the immune system, leading to chronic inflammation or immune exhaustion, both of which contribute to lymphopenia.

 4. Medications used to manage side effects may also lower lymphocytes

Immunosuppressive medication can be prescribed to control side effects caused by cancer treatment. While helpful, these medications can also suppress the immune system and reduce blood lymphocyte count.

Why this matters

Lymphopenia during cancer treatment isn’t just a side effect—it can also be a sign of how the body is responding to treatment. Studies suggest that patients who develop persistent or severe lymphopenia may have:

• A higher risk of infection
• Worse treatment outcomes
• Increased chances of severe immune-related side effects during immunotherapy

How is lymphopenia related to immunotherapy side effects?

ICIs have revolutionised cancer treatment. These powerful medications help the body’s immune system fight tumours more effectively. But sometimes, the immune system becomes too active, and instead of just attacking cancer cells, it mistakenly targets healthy organs.

This is known as immune-related adverse events (irAEs). These side effects can affect the skin, lungs, liver, intestines, thyroid, and other organs, sometimes even becoming life-threatening.

Where does lymphopenia fit in?

Research suggests that lymphopenia (e.g., a low blood lymphocyte count) could be an early warning sign of these immune-related side effects.

Here’s how:

• Immune exhaustion: ICIs activate T cells (e.g., a type of lymphocyte), but overactivation can lead to exhaustion or destruction of these cells, dropping their numbers in the blood
• Inflammatory damage: When the immune system begins attacking healthy tissues, the inflammation can also damage white blood cells, further reducing lymphocyte counts
• Severity marker: Studies show that patients who develop lymphopenia during ICI therapy are more likely to experience severe or multiple side effects, such as pneumonitis (lung inflammation), colitis (bowel inflammation), or hepatitis

Why does this matter?

While lymphopenia may not directly cause side effects, it can serve as a warning sign that the immune system is overreacting. If doctors detect low lymphocyte levels during treatment, they may:

• Monitor the patient more closely
• Adjust the immunotherapy dose
• Use other medications to calm the immune system before more serious damage occurs

By detecting these signs early on, doctors can reduce complications, prevent hospitalisation, and keep treatment on track.

Real-world insight

A 2022 study on melanoma patients receiving ICIs found that those who developed lymphopenia during treatment had significantly shorter survival rates and higher rates of immune-related toxicity compared to those with normal lymphocyte counts.

Another recent 2024 meta-analysis showed that lymphopenia was associated with a higher risk of severe immune-related side effects across multiple cancer types, including lung and kidney cancer.

What are the signs and symptoms to watch for?

When a patient is undergoing ICI therapy, monitoring for early signs of immune-related complications is essential. One warning sign is lymphopenia, or a low blood lymphocyte count. While lymphopenia itself may not always cause noticeable symptoms, it can indicate that the immune system is under stress or becoming overactive, which increases the risk of immune-related adverse events (irAEs).

General symptoms associated with lymphopenia or immune suppression

Lymphopenia is typically monitored through routine blood tests, but patients may also begin to experience symptoms such as:

• Fatigue or general weakness
• Frequent infections (such as colds, respiratory infections, or urinary tract infections)
• Mouth ulcers or sores
• Swollen lymph nodes
• Persistent fever or chills
• Unexplained weight loss or reduced appetite

These symptoms may appear gradually and should not be ignored, especially during cancer treatment.

Symptoms of Immune-Related Adverse Events (irAEs)

Immune checkpoint inhibitors can overstimulate the immune system, causing it to attack healthy tissues and organs. Lymphopenia may be a sign that these side effects are occurring. Some of the more common organ-related symptoms to monitor include:

• Shortness of breath or dry cough (possible pneumonitis)
• Diarrhoea, abdominal pain, or blood in stool (possible colitis)
• Yellowing of the skin or eyes (possible hepatitis)
• New or worsening skin rashes, itching, or peeling
• Headaches, blurred vision, confusion, or seizures (possible neurologic involvement)
• Muscle or joint pain
• Chest pain or irregular heartbeats (possible myocarditis)

Can doctors prevent these side effects?

Immune checkpoint inhibitors (ICIs) have significantly improved treatment outcomes for many cancer patients. However, they can also cause an adverse set of immune-related side effects. Fortunately, most of these problems can be avoided or dealt with, especially if detected early on. To lower risk and improve treatment safety, doctors employ a combination of patient education, close monitoring, timely intervention, and prompt clinical action.

Monitoring blood counts and immune function

One of the key tools for early detection is the complete blood count (CBC) test. This simple test can identify lymphopenia, or a drop in lymphocyte levels, which may serve as an early warning sign of immune system overactivation or treatment-related stress.

By regularly checking blood cell levels, doctors can:

• Detect immune-related toxicity before symptoms become severe
• Adjust the treatment dose or schedule
• Pause or discontinue therapy temporarily, if needed

In many cases, a drop in lymphocyte count occurs before the appearance of clinical symptoms, which makes proactive monitoring especially valuable.

Managing side effects with medications

If early signs of immune-related side effects appear, doctors may prescribe medications to calm the immune response. These may include:

• Corticosteroids (such as prednisone) to reduce inflammation
• Immunosuppressive drugs (like infliximab or mycophenolate) for more severe reactions
• Supportive therapies such as antibiotics, antidiarrheal agents, or intravenous fluids

These treatments can help manage symptoms while allowing the patient to safely continue or resume cancer therapy.

Personalised care and treatment adjustments

Not every patient reacts the same way to immunotherapy. As a result, doctors carefully assess each patient’s:

• Cancer type and stage
• Pre-existing medical conditions
• Blood test results
• History of autoimmune disorders or infections

This individualised approach allows clinicians to adjust therapy plans based on risk factors. In high-risk patients, close monitoring and preventive measures may be implemented from the start.

The role of patient education

Educating patients about potential side effects and the importance of reporting symptoms early on is a critical part of prevention. Patients are encouraged to communicate with their healthcare team about any new or unusual changes in their health—even mild symptoms like fatigue, cough, or skin rashes.

Early detection and prompt action can prevent complications from escalating and help maintain the balance between effective cancer treatment and patient safety.

Summary 

Immunotherapy has transformed the way we treat cancer, offering many patients longer lives and new hope. However, like any powerful treatment, it comes with risks. Among the early warning signs, doctors are learning to monitor patients for lymphopenia, a drop in the body’s infection-fighting white blood cells (e.g., lymphocytes).

While lymphopenia may not cause symptoms right away, research shows that it is a strong indicator that the immune system is under stress, especially in patients receiving ICIs. These innovative drugs are designed to help the immune system recognise and destroy cancer cells, but in some cases, they can also trigger it to attack healthy tissues, leading to side effects in organs such as the lungs, liver, skin, or intestines.

In patients with lymphopenia, early detection is key. With regular blood tests, open communication between patients and healthcare providers, and quick action when symptoms arise, many of these complications can be managed or even prevented.

Understanding lymphopenia doesn’t just help doctors: it empowers patients to take a more active role in their care, ask the right questions, and feel more confident as they navigate their treatment journey.