CELLS OF THE IMMUNE SYSTEM
, Cont,
• Various white blood cells, or leukocytes, that
participate in the immune response are carried within
the blood and lymph and populate the lymphoid organs.
• Only the lymphocytes possess the attributes displayed
by adaptive immune response of diversity, specificity,
memory, and self/nonself recognition. All the other cells
play accessory roles in adaptive immunity, serving to
activate lymphocytes, to increase the effectiveness of
antigen clearance by phagocytosis, or to secrete various
immune-effector molecules.
• Some leukocytes, especially T lymphocytes, secrete
cytokines that act as immunoregulatory hormones and
play important roles in the regulation of immune
responses.
, Hematopoiesis
• Hematopoiesis is the formation and development of red and white
blood cells.
• All blood cells arise from the hematopoietic stem cell (HSC). Stem
cells are cells that can differentiate into other cell types; they are self-
renewing—they maintain their population level by cell division. In
humans, hematopoiesis begins in the embryonic yolk sac during the
first weeks of development. Here, yolk-sac stem cells differentiate into
primitive erythroid cells that contain embryonic hemoglobin.
• In the third month of gestation, hematopoietic stem cells migrate
from the yolk sac to the fetal liver and then to the spleen; these two
organs have major roles in hematopoiesis from the third to the
seventh months of gestation.
• After that, the differentiation of HSCs in the bone marrow becomes
the major factor in hematopoiesis, and by birth there is little or no
hematopoiesis in the liver and spleen. Every functionally specialized,
mature blood cell is derived from the same type of stem cell.
, • A hematopoietic stem cell is multipotent, or pluripotent, able to
differentiate in various ways and thereby generate erythrocytes,
granulocytes, monocytes, mast cells, lymphocytes, and
megakaryocytes. These stem cells are few, normally fewer than
one HSC per 5 x 104 cells in the bone marrow.
• Properties of stem cell
i. They have the capacity to give rise to more differentiated
cells.
ii. They are self-renewing, because each division of a stem cell
creates at least one stem cell.
• Hematopoietic stem cells are maintained at stable levels
throughout adult life; however, when there is an increased
demand for hematopoiesis, HSCs display an enormous
proliferative capacity.
, Cont,
• Various white blood cells, or leukocytes, that
participate in the immune response are carried within
the blood and lymph and populate the lymphoid organs.
• Only the lymphocytes possess the attributes displayed
by adaptive immune response of diversity, specificity,
memory, and self/nonself recognition. All the other cells
play accessory roles in adaptive immunity, serving to
activate lymphocytes, to increase the effectiveness of
antigen clearance by phagocytosis, or to secrete various
immune-effector molecules.
• Some leukocytes, especially T lymphocytes, secrete
cytokines that act as immunoregulatory hormones and
play important roles in the regulation of immune
responses.
, Hematopoiesis
• Hematopoiesis is the formation and development of red and white
blood cells.
• All blood cells arise from the hematopoietic stem cell (HSC). Stem
cells are cells that can differentiate into other cell types; they are self-
renewing—they maintain their population level by cell division. In
humans, hematopoiesis begins in the embryonic yolk sac during the
first weeks of development. Here, yolk-sac stem cells differentiate into
primitive erythroid cells that contain embryonic hemoglobin.
• In the third month of gestation, hematopoietic stem cells migrate
from the yolk sac to the fetal liver and then to the spleen; these two
organs have major roles in hematopoiesis from the third to the
seventh months of gestation.
• After that, the differentiation of HSCs in the bone marrow becomes
the major factor in hematopoiesis, and by birth there is little or no
hematopoiesis in the liver and spleen. Every functionally specialized,
mature blood cell is derived from the same type of stem cell.
, • A hematopoietic stem cell is multipotent, or pluripotent, able to
differentiate in various ways and thereby generate erythrocytes,
granulocytes, monocytes, mast cells, lymphocytes, and
megakaryocytes. These stem cells are few, normally fewer than
one HSC per 5 x 104 cells in the bone marrow.
• Properties of stem cell
i. They have the capacity to give rise to more differentiated
cells.
ii. They are self-renewing, because each division of a stem cell
creates at least one stem cell.
• Hematopoietic stem cells are maintained at stable levels
throughout adult life; however, when there is an increased
demand for hematopoiesis, HSCs display an enormous
proliferative capacity.
