- B cells
Also known as B lymphocytes, these are the cells that, along with T cells, constitute the adaptive immune system. Upon activation by an encounter of antigen, they differentiate into plasma cells, which produce antibodies. In birds, antibodies are produced inside the Bursa fabricii, hence their name. In humans, they are made in the bone marrow. Humans have about 10 billions different types of B cells capable of recognizing different antigens.