The Hemopoietic System
The great majority of cells involved in mammalian immunity are derived from
precursors in the bone marrow (left half of figure 1) and circulate in the blood,
entering and sometimes leaving the tissues when required. A very rare stem cell
persists in the adult bone marrow (at a frequency of about 1 in 100 000 cells), and
retains the ability to differentiate into all types of blood cell. Hemopoiesis has been
studied either by injecting small numbers of genetically marked marrow cells into
recipient mice and observing the progeny they give rise to (in vivo cloning) or by
culturing the bone marrow precursors in the presence of appropriate growth factors
(in vitro cloning). Proliferation and differentiation of all these cells is under the
control of soluble or membrane bound growth factors produced by the bone marrow
stroma and by each other.
Within the cell these signals switch on specific transcription factors, DNA-binding
molecules which act as master switches that determine the subsequent genetic
programme, in turn giving rise to development of the different cell types (known as
lineages). Remarkably, recent studies have shown that it is possible to turn one
differentiated cell type into another by experimentally introducing the right
transcription factors into the cell. This finding has important therapeutic implications,
e.g. in curing genetic immunodeficiencies. Most hemopoietic cells stop dividing once
they are fully differentiated. However, lymphocytes divide rapidly and expand
following exposure to antigen. The increased number of lymphocytes specific for an
antigen forms the basis for immunological memory.
