Overview

Monocytes are a type of white blood cell that play a crucial role in our immune system. They identify, engulf and digest pathogens that have invaded our bodies, aid in the body's inflammatory response and contribute to numerous aspects of tissue repair.¹ Monocytosis is defined by an elevated number of these cells in our bloodstream, specifically a count between 800 to 1000 cells per mm³.²

While this increased number of cells might seem beneficial from an immune standpoint, their excess can negatively impact a person's overall health in several ways.³ This article explores these impacts alongside the causes of this condition, its symptoms and how it is diagnosed and treated. 

Causes 

Monocytosis is usually associated with a chronic infection or disease that the body is fighting. Common causes include:³

• Chronic Infections: When the body detects an infection, it increases the production of monocytes. In chronic infections, the body continues to produce elevated levels of these cells, which can result in their accumulation in the blood¹
• Autoimmune Diseases: In autoimmune diseases, the immune system mistakenly attacks healthy cells. This causes inflammation which in turn attracts monocytes and increases their numbers in the bloodstream
• Blood Disorders: Certain blood disorders disrupt normal monocyte production. When the body attempts to compensate for these disruptions, it can overproduce monocytes, leading to abnormally high levels
• Certain Cancers: Some cancers, like leukaemia, interfere with blood cell production in the bone marrow. The body's attempt to manage these changes can increase monocyte numbers
• Tissue  Damage: Significant tissue damage, such as a heart attack, can trigger inflammatory responses and the release of monocytes from the bone marrow

Symptoms 

If a person's monocyte count is high, they won’t necessarily present with symptoms. Instead, any symptoms they experience are likely a result of the underlying condition that caused their count to become high in the first place.⁴ However, there are some general symptoms that people can experience if they have monocytosis. These include: 

• Fever
• Swelling
• Fatigue 
• Weakness
• Unexplained weight loss

Impact on Overall Health  

An increased number of monocytes leads to an overactive immune response. This response can result in chronic inflammation throughout the body which in turn affects various different bodily systems.³

Cardiovascular Health

An increased number of monocytes can accelerate the development of atherosclerosis, a condition that occurs when plaque builds up in the arteries that supply the heart with blood.⁴ The inflammation associated with monocytosis can independently damage blood vessel walls, making them more susceptible to plaque buildup. As such, monocytosis indirectly leads to a higher risk of heart disease, heart attacks, and strokes.⁴

Metabolic Health

Metabolic processes are the chemical reactions that occur in our bodies to produce energy and build essential compounds for growth and maintenance. Altered monocyte populations can disrupt these processes, contributing to insulin resistance and increasing the risk of type 2 diabetes.⁵ Additionally, chronic inflammation linked to monocytosis can impair blood sugar regulation and metabolism, leading to issues like someone suffering to maintain a consistent weight. 

Respiratory Health

An increased number of monocytes can lead to excessive inflammation in the lungs. This inflammation can make existing respiratory conditions like asthma worse but also contribute to the development of interstitial lung disease.⁶ This disorder leads to breathing difficulties and a general reduction in lung function over time.

Neurological Health

Under normal conditions, the brain is protected from inflammation by the blood-brain barrier. This barrier network of blood vessels and tissue that acts as a filter closely regulates the passage of molecules between the bloodstream and the brain, allowing only essential nutrients and compounds to enter while blocking harmful substances. In cases of monocytosis, the integrity of this barrier can be compromised. This can increase the brain's vulnerability to inflammation. This neuroinflammation has been associated with a higher risk of neurodegenerative diseases such as Alzheimer's and Parkinson's⁷ and has also been reported to contribute to cognitive decline and mental health disorders, including depression and anxiety.⁸

Gastrointestinal Health

Monocytes play a role in maintaining the integrity of the intestinal barrier, which regulates the passage of substances between the gut and the bloodstream. In monocytosis, an overabundance of monocytes can disrupt this balance. This can lead to increased intestinal permeability, commonly known as "leaky gut syndrome”.⁹ This increased permeability also increases the chance of harmful substances entering the bloodstream, which in turn can cause a number of other health problems.⁹

Diagnosis

Monocytosis can be diagnosed through a series of tests. The most common of these is called a complete blood count (CBC).¹⁰ This test counts the number and types of different types of blood cells, including monocytes, present in a sample of blood. If the CBC shows an elevated monocyte count, further tests are conducted. One of these tests is a peripheral blood smear.¹⁰ In this test, a sample of blood is examined under a microscope and the size and shape of cells are analysed to reveal potential clues about the cause of monocytosis. Healthcare providers also often calculate a person's monocyte count. Depending on the findings and conclusions of these tests, imaging such as X-rays, CT scans or MRIs may also be carried out to help identify the underlying condition or conditions causing monocytosis.¹⁰

Treatment 

Because monocytosis is not a condition itself but rather the body's response to an underlying issue, its treatment focuses on addressing the root cause of the elevated count rather than the count itself.³

Antibiotics are commonly used to treat bacterial infections. The choice of antibiotic depends on several factors, including the type of bacteria, the severity of the infection and any allergies a patient may have. Antibiotics target and eliminate bacterial pathogens, reducing the need for an immune response. As the body no longer needs to mount a defence against the bacterial infection, the monocyte count will decrease.³

Antiviral and antifungal medications combat viral and fungal pathogens, respectively. Like bacterial infections, once the pathogen is eradicated, the monocyte count will return to normal levels.³ Antiviral medications prevent the virus from replicating, while antifungal medications target and eliminate fungal pathogens. 

The treatment of autoimmune disorders typically involves anti-inflammatory drugs or immune-suppressing medications.¹¹ These medications dampen the immune system's response, which in turn reduces inflammation and signals the body to produce fewer monocytes.

The treatment of cancers is complex and depends on various factors, such as the type, stage, and location of the cancer. Common treatments include chemotherapy, radiotherapy, and surgeries to remove cancerous tissue.¹² As the underlying condition is controlled by these treatments, monocyte levels typically decrease, restoring counts to a normal level.

It is essential to monitor monocyte counts during treatment to ensure that the underlying cause is being managed properly.³ If a person's count remains elevated even after intervention, further evaluation and alternative treatment are usually needed. 

Summary 

Monocytosis is a condition characterised by elevated monocyte levels in the blood. Rather than a disease itself, monocytosis instead serves as an indicator of underlying health issues. Chronic monocytosis can have far-reaching effects on a person's health, including their cardiovascular, respiratory, neurological and metabolic systems. The timely and accurate diagnosis of monocytosis and its underlying causes are essential for effectively managing it. By addressing the primary condition causing the high count and continuously monitoring levels during treatment, doctors can reduce the risk of long-term health complications and improve patient outcomes.