The cells of the adaptive immune response (B and T lymphocytes) are powerful players in the immune system. Each lymphocyte creates a unique receptor for recognition of pathogens during precursor differentiation in bone marrow or thymus. Together, this results in a large repertoire of antigen receptors with the potential to recognize many different pathogens specifically. On top of this broad repertoire, the lymphocytes that actually recognize antigen are capable of undergoing enormous clonal proliferation, thereby generating huge numbers of daughter cells with the potential to recognize the same pathogen. This clonal expansion generates effector cells for a strong response and long-term memory in the form of memory B and T cells and immunoglobulin (Ig)-producing plasma cells.