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.
According to About.com, the B-cell lymphocytes contained in the lymph nodes activate when an antigen is present, and antibodies are produced to fight that particular antigen. Additionally, the T-cell lymphocytes also aid in the destruction of these harmful pathogens and promote cell-mediated immunity.