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.
The activity of B cells and T cells targets specific antigens. This means that each time a new kind of antigen invades the body, the immune system must produce a new round of B cells and T cells, which attack only that antigen. It is estimated that the immune system can create more than 100 million types of antibodies.