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 are activated when a particular phagocyte known as an antigen-presenting cell (APC) displays the antigen to which the T cell is specific. This blended cell (mostly human but displaying an antigen to the T cell) is a trigger for the various elements of the specific immune response. A subtype of T cell known as a T helper cell performs a number of roles. T helper cells release chemicals to.