Reactive lymphocytes are activated lymphocytes that appear during an active immune response. In healthy adults, they are generally absent or present only at very low levels on a manual differential.

Their presence most often reflects viral infection, recent immune stimulation, or sometimes medication reactions. They are part of a normal adaptive immune response, not inherently a sign of malignancy. In a healthy person, a small number of reactive lymphocytes usually indicates that the immune system is responding appropriately to some antigenic trigger. The key distinction is whether the cells are reactive and temporary versus persistent and morphologically concerning.