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.
1 A CD4 + T cell that recognizes the displayed antigen is stimulated to release lymphokines. 2  These, in turn, stimulate the B cell to enter the cell cycle. 3  Because of the part they play in stimulating B cells, these CD4 + T cells are called Helper T cells ( Th ). A CD4 + T cell that recognizes the displayed antigen is stimulated to release lymphokines. 2  These, in turn, stimulate the B cell to enter the cell cycle. 3  Because of the part they play in stimulating B cells, these CD4 + T cells are called Helper T cells ( Th ).