Immune Cells and Organs

Immune cells are leukocytes (white blood cells) and, together with red blood cells, are ultimately derived from the same precursor or progenitor cells in the bone marrow. As illustrated in Figure 1, these stem cells give rise to either lymphoid or myeloid progenitors that subsequently differentiate into the different immune cells. A few other types of immune cells, including NK cells and mast cells, also arise from these pluripotent stem cells, but the pathways of their development are not fully known.

The differentiation of lymphocytes takes place in the central (also called primary) lymphoid organs—that is, bone marrow in the case of B cells and thymus in the case of T cells. After puberty, the thymus gradually atrophies and the production of new T cells decreases. After their maturation in the primary lymphoid organs, both types of lymphocytes migrate from these tissues

Lymphoid progenitor

Pluripotent stem cell

Lymphoid progenitor

Pluripotent stem cell

B cell

T cell

Neutrophil

Monocyte

Figure 1 Immune cells give rise to either lymphoid or myeloid progenitors, which subsequently differentiate into the different immune cells.

B cell

T cell

Granulocytes or

Ploymorphonuclear

Leukocytes

Neutrophil

Monocyte

Table 1

Functions of myeloid and lymphoid immune cells

Cell

Cell type

Function

progenitor

Myeloid

Basophil

Unknown

Eosinophil

Killing of antibody-coated parasites

Neutrophil

Phagocytosis and activation of bactericidal mechanisms

Macrophage3

Phagocytosis

Mast cell

Release of histamine and other vasoactive and inflammatory mediators

Lymphoid

B cell

Antibody production

T cell

Cytotoxic

Killing of virus-infected cells

Helper (Th1

Activation of other cells such as

and Th2)

B cells and macrophages

Natural Killer

Killing of virus-infected cells and

cells

cancer cells

aThis cell type circulates as monocytes in the bloodstream and matures into macrophages when taking up residence in a tissue.

Figure 1 Immune cells give rise to either lymphoid or myeloid progenitors, which subsequently differentiate into the different immune cells.

aThis cell type circulates as monocytes in the bloodstream and matures into macrophages when taking up residence in a tissue.

through the bloodstream into the peripheral, or secondary, lymphoid tissues. These include the lymph nodes, spleen, and lymphoid tissues associated with mucosa, and they constitute the main sites at which the reaction of B and T lymphocytes with foreign antigens takes place. Lymph then carries circulating lymphocytes from the peripheral lymphoid tissues to the thoracic duct, where they reenter the bloodstream. The functions of the various immune cells are summarized in Table 1.

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