What is the most likely cause of elevated WBC count with reactive lymphocytes in an otherwise healthy 24-year-old?

Elevated white blood cell (WBC) counts with the presence of reactive lymphocytes are often indicative of a viral infection. In otherwise healthy individuals, particularly younger adults, the immune system responds to viral pathogens by increasing the number of lymphocytes, which are a type of white blood cell that play a crucial role in the immune response. Reactive lymphocytes, which may appear larger and more abundant than typical lymphocytes, suggest that the body is actively responding to a viral infection, such as infectious mononucleosis or other viral illnesses.

While bacterial infections can also cause an increase in WBC count, they typically result in neutrophilia (an increase in neutrophils) rather than lymphocytic predominance. Allergic reactions usually elevate eosinophils and do not primarily involve a significant lymphocytic response. Chronic inflammatory conditions may also lead to elevated WBC counts, but these conditions typically involve a more complex picture of immune response, often with varying types of leukocyte involvement rather than a specific increase in reactive lymphocytes.

Thus, the context provided by the age of the individual, their otherwise healthy status, and the specific pattern of lymphocyte elevation strongly points to a viral cause as the most likely explanation.