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 immune system is a collection of billions of cells that travel through the bloodstream. They move in and out of tissues and organs, defending the body against foreign bodies (antigens), such as bacteria, viruses and cancerous cells. There are two types of lymphocytes : B cells-produce antibodies which are released into the fluid surrounding the bodyâs cells to destroy the invading viruses and bacteria. T cells (see picture opposite) - if the invader gets inside a cell, these (T cells) lock on to the infected cell, multiply and destroy it.