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.
There are four types of T cells, each with a special purpose. Helper T cells detect infection and get the other cells of the immune system ready for battle. Helper T cells also tell B cells to produce antibodies, highly specialized proteins that help defend the body against infection. Cytotoxic, or killer, T cells destroy infected cells. Regulatory T cells tell the immune system when the battle with the pathogens is over and to stop fighting. Memory T cells remember how to defeat an infection and can respond rapidly if the same infection reoccurs.