The term
myeloid ( + ) is an adjective that in its broadest
sense means either "pertaining to
bone marrow" or "resembling bone marrow", and the related adjective
myelogenous ( + ) means "arising from bone marrow". In
hematopoiesis, both terms refer to
blood cells that arise from a
progenitor cell for
granulocytes,
monocytes,
erythrocytes, or
platelets (the
common myeloid progenitor or CMP) and often even more specifically to the lineage of the
myeloblast (the
myelocytes,
monocytes, and their daughter types); thus, although all blood cells, even
lymphocytes, are born in the bone marrow,
myeloid cells in the narrowest sense of the term can be distinguished from
lymphoid cells, that is, lymphocytes, which come from common lymphoid progenitor cells (CLPs) that give rise to
B cells and
T cells. Those cells'
differentiation (that is,
lymphopoiesis) is not complete until they migrate to
lymphatic organs such as the
spleen and
thymus for programming by antigen challenge. Thus, among
leukocytes, the term
myeloid is associated with the
innate immune system, in contrast to
lymphoid, which is associated with the
adaptive immune system. Similarly,
myelogenous usually refers to nonlymphocytic white blood cells, and
erythroid can often be used to distinguish "erythrocyte-related" from that sense of
myeloid and from
lymphoid. The word
myelopoiesis has several senses in a way that parallels those of
myeloid, and myelopoiesis in the narrower sense is the regulated formation specifically of myeloid leukocytes (myelocytes), allowing that sense of
myelopoiesis to be contradistinguished from
erythropoiesis and
lymphopoiesis (even though all blood cells are born in the marrow).