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Why does infectious mononucleosis cause unusually large lymphocytes to appear in the blood?

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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.

Infectious mononucleosis, which is caused by the Epstein-Barr virus, is characterized by the appearance of unusually large lymphocytes in the blood, often referred to as atypical lymphocytes.12

These enlarged cells are not a sign of malignancy but rather represent a specialized component of the body's complex defense mechanism against the viral infection. As the immune system mounts this response to combat the virus, these atypical cells circulate in the blood; they typically disappear once the active phase of the infection subsides.12