The immune system relies on specialized cells to detect and neutralize foreign invaders. White blood cells, also known as leukocytes, are the body’s primary defense force against infectious agents, including viruses. Viral infections are well-known to disrupt the normal balance of cells circulating in the blood. Research into SARS-CoV-2, the virus responsible for COVID-19, has established a clear link between infection and changes in these protective cell levels. Understanding this relationship helps clarify how the virus impacts the body’s ability to defend itself.
What Are White Blood Cells and Their Role?
White blood cells (WBCs), or leukocytes, are circulating components of the blood produced primarily in the bone marrow. These cells are fundamentally responsible for protecting the body from pathogens, such as bacteria, parasites, and viruses. They constantly patrol the bloodstream and tissues, acting as first responders to any perceived threat.
There are five major types of WBCs, each with distinct functions in the immune response: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils and monocytes are part of the innate immune system, offering a rapid, non-specific defense. Lymphocytes, which include T cells and B cells, form the adaptive immune system, providing targeted, long-lasting immunity.
COVID-19’s Effect on Overall WBC Count
A decrease in the total WBC count, a condition medically termed leukopenia, is a documented finding in a significant portion of individuals infected with SARS-CoV-2. This observation is contrary to many bacterial infections, which typically cause an increase in overall WBC counts.
Leukopenia is a common feature of COVID-19 infection, and the likelihood of a low WBC count tends to rise as the severity of the illness increases. However, a high total WBC count, called leukocytosis, is also frequently observed in the most severe cases and is often a marker for a poorer clinical outcome. The specific pattern of which cell types are reduced or increased determines the overall count.
Specific Immune Cells Affected
The most characteristic and consistently observed change in the blood cell profile of COVID-19 patients is a marked reduction in the number of lymphocytes, a condition known as lymphopenia. Lymphocytes, which include T cells and B cells, are fundamental for developing a targeted, adaptive immune response to the virus. T cells destroy infected cells and coordinate the immune response, while B cells produce virus-neutralizing antibodies.
The depletion of T cells, particularly helper T cells (CD4+) and killer T cells (CD8+), is common and often correlates with disease severity. This loss severely impairs the body’s ability to clear the virus and control the infection. Conversely, the count of neutrophils is often maintained or elevated, leading to an abnormal ratio of neutrophils to lymphocytes that serves as a strong indicator of disease prognosis.
Other WBC types, such as monocytes and eosinophils, have also been observed to decrease in some patients. The persistent loss of lymphocytes signals a failure of the adaptive immune system to effectively control the viral load.
Biological Mechanisms of Reduction
The decrease in white blood cell counts, particularly lymphocytes, is a multi-faceted process driven by the virus and the body’s own hyper-reactive immune response. One mechanism involves the migration and sequestration of these cells out of the bloodstream and into inflamed tissues. Lymphocytes and monocytes are recruited in large numbers to sites of infection, such as the lungs, where they are needed to fight the virus, leading to lower numbers in the circulating blood.
Another mechanism is the massive induction of programmed cell death, or apoptosis, in immune cells. The intense inflammatory environment triggered by the infection causes many T cells to die off prematurely. This cell death is amplified by the presence of a “cytokine storm,” an excessive and uncontrolled release of inflammatory signaling molecules like Interleukin-6 (IL-6) and Tumor Necrosis Factor-alpha (TNF-α).
The systemic inflammatory response caused by the cytokine storm can also suppress the production of new immune cells in the bone marrow, a process called aborted lymphopoiesis. Research suggests that the SARS-CoV-2 virus may directly infect some immune cells. Certain types of neutrophils, known as granulocytic myeloid-derived suppressor cells (G-MDSCs), can also expand during severe infection and actively suppress the proliferation of T lymphocytes, further contributing to the state of lymphopenia.