Which symptom indicates a specific response to an infection?

Leukocytosis is a specific response to an infection characterized by an elevated white blood cell count. This increase in leukocytes is typically a direct response to pathogens in the body, such as bacteria or viruses, as the immune system activates to fight off the infection. The presence of elevated white blood cells can indicate the body is mounting an immune response, and different types of leukocytes may suggest specific types of infections—for example, neutrophilia often suggests a bacterial infection, while lymphocytosis may indicate a viral infection.

In contrast, the other symptoms listed, such as fatigue, appetite loss, and insomnia, are more generalized signs of illness and can be associated with a wide range of conditions—both infectious and non-infectious. They do not specifically reflect the biological immune response to infection in the same way that leukocytosis does. Therefore, leukocytosis stands out as a direct indicator of an acute response by the immune system to an infectious agent.