The immune system develops from hematopoietic precursor cells in the bone marrow. These precursors go through rounds of cell division and differentiation to give rise to the various different lymphoid and non-lymphoid lineages that go to make up the innate and adaptive immune systems. Myeloid cells of the innate immune system develop under the control of a variety of hematopoietic growth and differentiation factors which are orchestrated to result in the continuous production of the appropriate amounts of each cell type. Lymphoid cells of the adaptive immune system also start their development in the bone marrow. B lymphocytes responsible for the eventual production of antibodies develop along a pathway that requires rearrangement of the immunoglobulin genes to generate the vast array of individual B cell specificities and indeed, the normal progression of B cell development is regulated by the expression of immunoglobulin molecules. The other major component of the adaptive immune system, the T lymphocyte, develops in the thymus from precursor cells produced in the bone marrow. T cell development also requires rearrangement of genes that encode antigen specificity and selective events in the thymus ensure that mature T cells normally have the capacity to recognize foreign pathogen without recognizing self antigens.