Final answer:
Regulatory T cells (Treg), which are part of the lymphocytes family, are responsible for suppressing overactive T cells to maintain immune system balance.
Step-by-step explanation:
The white blood cell that suppresses overactive T cells is the Regulatory T cell (Treg), also known as suppressor T cells. These cells play a critical role in maintaining immune balance by preventing the immune system from overreacting, which can lead to autoimmune diseases. The lymphocytes category of white blood cells includes Regulatory T cells along with other types of T cells and B cells.
Regulatory T cells are distinguished by the expression of CD4, CD25, and FOXP3 molecules, and they have a pivotal role in suppressing other T cell immune responses. Granulocytes such as neutrophils, eosinophils, and basophils act primarily in fighting infections and facilitating allergic reactions, while monocytes differentiate into macrophages and dendritic cells to respond to infections or injuries.