While the tadpoles of competing species may complete metamorphosis in as little as just a few short months, Bullfrog tadpoles can remain in the same state for as long as one to two years before they actually become adult frogs.
Two types of white blood cells called lymphocytes are vital to the specific immune response. Lymphocytes are produced in the bone marrow, and mature into one of several subtypes. The two most common are T cells and B cells. An antigen is a foreign material that triggers a response from T and B cells.
T-cells, also called thymus cells or thymocytes, are a type of white blood cell called lymphocytes. They help fight against disease and infection. T-cells cells play a vital role in cell-mediated immunity. Once activated, T-cells secrete chemical messengers called cytokines, which stimulate immune system cells to engulf foreign substances (such as bacteria, viruses, fungi, and allergens) that enter the body. T-cells are continually produced in the bone marrow.