Here's why:

* T cells are a type of white blood cell that mature in the thymus gland. They are responsible for cell-mediated immunity, which means they directly attack infected or cancerous cells.

* B lymphocytes (B cells), the other major type of lymphocyte, primarily reside in the lymph nodes and spleen. They produce antibodies that target specific antigens in the bloodstream.

While both T and B cells contribute to immune responses, T cells are the ones that actively circulate through the blood and tissues, constantly scanning for threats.