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Why do viral infections often cause lymphocytosis?

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Illustration of a blood vessel with a cutaway view showing red blood cells, white blood cells, platelets, and plasma, each labeled to highlight the main components of blood. — white blood cell Blood is made up of multiple components, including red blood cells, white blood cells, platelets, and water-rich plasma.

Viral infections often cause lymphocytosis—an increase in the number of small lymphocytes in the blood—because these cells are central to the body's immune system response against viruses.12

When a virus enters the body, it often infects cells, causing viral antigens to appear on the cell membrane. T lymphocytes and B lymphocytes recognize these foreign antigens and are stimulated to multiply into clones of identical cells to combat the infection. This proliferation is a key part of the defense mechanism against the pathogen.2341

In some cases, such as infectious mononucleosis, the immune response is particularly robust, leading to the appearance of large, atypical lymphocytes in the circulation as the body works to neutralize the virus. Once the infection subsides, these elevated lymphocyte counts typically return to normal as the immune system resolves the threat.12