The marrow thus contains competent B cells in addition to all of the developing progenitors. B cell development involves the process of immunoglobulin gene rearrangements which was discussed in the preceding lecture. Cells at various stages in this progression can be identified by a combination of cytoplasmic and cell surface markers. Temporally speaking, the heavy chain rearranges first followed by the light chain. The latter cell is a fully developed, virgin immunocompetent B lymphocyte.
T cells develop in an organ called the thymus.
Stem cells which are committed to the lymphoid lineage migrate from the bone marrow to the thymus under the influence of one or more chemotactic factors secreted by the thymus. The thymus performs the dual role of both directing the development of the T cells and also of deleting those cells that express self recognition capability. We will discuss this process in more detail in a later lecture. As with B cells, the development of T cells can be followed through stages based on receptor gene rearrangements and cell surface markers. The earliest cells which are recognizable as T cells are those which have begun to rearrange their antigen receptor genes.