Cytokines: Interleukins
117
Hematopoiesis
The process of populating and replacing circulating erythrocytes and leukocytes from
stem cells contained within the bone marrow.
Macrophage
Phagocytic leukocyte found in various tissues, which is important in nonspeciFc
cellular immunity and antigen presentation, and which is a major producer of and
responder to interleukins.
T lymphocyte (T cell)
A type of white blood cell capable of responding to foreign antigens and thus of
mediating cellular immunity, which it does by secreting a variety of interleukins and
other cytokines to activate leukocytes and other cells.
Homolog
A structurally related interleukin molecule whose activity is mediated by a receptor that
is common to all homologs within an interleukin family.
Paralog
A structurally related interleukin molecule whose activity is mediated by a receptor
distinct from those of other paralogs within an interleukin family.
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The
harmonious
regulation
of
vital
physiological
processes,
for
example,
replenishment of mature blood cells from bone marrow stem cells, termed
hematopoiesis
, and the activation of defense mechanisms against pathological
microbes and injury has been shown to depend on the production and action
of a variety of secreted biologically active proteins, collectively known as cytokines.
Central among cytokines is a class of mediators, largely involved in the regulation
of immune, inflammatory, and hematopoietic functions, designated interleukins
primarily on the basis that interleukins are produced mainly by leukocytes and act
locally on other leukocytes in surrounding tissues. Each interleukin, of which there
are now 29 designated ones, exercises a spectrum of biological activities via speciFc
cell surface receptors. Overlaps of biological activities have been found to be common
among different interleukins and to result in many cases from the sharing of receptor
components. On binding their cognate interleukins, receptors activate intracellular
signaling pathways leading to the transcription of nuclear genes and expression
of proteins necessary to commit the cell to a number of contingent events and
responses according to the particular interleukin bound and cell type. Interleukins,
however, probably rarely act alone and
in vivo
form complex interactive networks
both among themselves and with other cytokines. Such complicated intercellular
communications systems have made it difFcult to precisely deFne the biological
roles of interleukins in health and disease. In certain diseases, for example, cancer,
some exogenously administered interleukins induce beneFcial responses, but it has
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