Immune Defense against Bacterial Pathogens: Adaptive or Acquired Immunity (page 2)
(This chapter has 6 pages)
© Kenneth Todar, PhD
The immunological System
The immunological system is comprised of the lymphoid
and organs of the body. Lymphoid tissues are widely distributed:
are concentrated in bone marrow, lymph nodes, spleen, liver, thymus,
Peyer's patches scattered in linings of the GI tract. The lymphoid
is encompassed by the system of mononuclear phagocytes.
are the predominant cells, but macrophages, dendritic cells, and
plasma cells are
also present. Lymphocytes are cells which circulate, alternating
blood stream and the lymphatic channels. The distribution of lymphatic
tissues that make up the immunological system in humans is illustrated
Figure 1. Anatomy of the
System. (A): The major components of the immunological system are lymph
connected by lymph ducts, Peyer's patches (masses of lymphocytes in the
lower gastrointestinal tract), thymus, spleen, and bone marrow. (B): A
lymph node. Afferent lymph ducts bring lymph-containing antigens into
lymph node. Macrophages, dendritic cells and B-cells in the
make contact with the antigen and process it for presentation to
B-cells and T- cells, thereby initiating an immune response. As a
B-cells are stimulated to develop into antibody-secreting plasma cells,
and T-cells are stimulated to develop into effector T cells of various
classes. Antibodies leave the lymph node by the efferent ducts that
into the blood stream. Lymphocytes can also leave the node by the
duct and travel to other sites in the lymphatic system or enter into
blood circulation. A single lymphocyte completes a circuit through the
circulating blood and lymphatic systems once every 24 hours.
Organs comprising the immune system
Bone Marrow All cells of
the immune system are initially
derived from the bone marrow. During hematopoiesis, bone marrow stem
cells develop into either mature cells or
precursors of cells that migrate out of the bone marrow to continue
their maturation elsewhere. The bone marrow produces lymphocytes
(B-cells, immature T-cells, and natural
killer cells), granulocytes (including neutrophils, monocytes and
dendritic cells), in addition to red
blood cells and platelets.
Thymus T-cells mature in the
thymus. Immature T-cells, also known as pre-T cells (or prothymocytes),
the bone marrow and migrate into the thymus. In a
maturation process sometimes referred to as "thymic education", T cells
that are beneficial to the immune system are spared, while T
cells that might evoke a detrimental autoimmune response are
eliminated. The mature T cells are then released into the bloodstream.
Spleen The spleen is is made up
of B cells, T cells, macrophages, dendritic cells,
natural killer cells and red blood cells. The spleen filters antigens
directly from the blood that passes through it, and migratory
macrophages and dendritic cells bring antigens to the
spleen via the bloodstream. An immune response is initiated when a
macrophage or dendritic cell "presents antigen" to appropriate B
or T cells. In the spleen, B cells become activated and produce large
amounts of antibody.
Lymph Nodes The lymphatic
system parallels the circulatory blood system. It is periodically
guarded by lymph nodes, which are
found throughout the body. Composed mostly of T cells, B cells,
dendritic cells and macrophages, the nodes drain fluid from most
tissues. Antigens are filtered out of the lymph in the lymph node
before returning the lymph to the circulation. In a similar fashion as
the spleen, macrophages and dendritic cells capture antigens
and present them T and B cells, consequently
initiating an immune response.
Figure 2. Origin and
differentiation of cells of the immune system.
Cells of the immune system
B-cells The major function of B
lymphocytes is to develop into antibody-secreting plasma cells
following stimulation by foreign antigens of bacteria,
viruses and tumor cells. Antibodies are specialized proteins that
specifically recognize and bind to specific antigens that caused their
production and binding to foreign antigens is often
critical as a means of signaling other cells to engulf, kill or remove
that substance from the body.
T-cells T lymphocytes are
usually divided into two major
subsets that are functionally and phenotypically
different. T helper (TH) cells, also called CD4+ T cells, are involved
in coordination and regulation of immunological responses. They
function to mediate responses by the
secretion of lymphokines that stimulate or otherwise affect other cells
involved in the immune responses.
The second subset type of T lymphocytes are cytotoxic T
lymphocytes ( Tc cells or CTLs) or CD8+ T
cells. These cells are involved in
directly killing certain tumor cells, virus-infected cells, transplant
sometimes eucaryotic parasites. CD8+ T cells are also important
down-regulation of immune responses.
Both types of T cells can be found
throughout the body, most conspicuously in lymphoid organs (lymph nodes
and spleen) but also the
liver, lung, blood, and the intestinal tract.
Natural Killer cells Natural
killer cells, known as NK cells,
are similar to CTLs (CD8+
T cells). They function as effector cells that directly kill certain
tumors such as melanomas, lymphomas and virus-infected cells, most
notably herpes and cytomegalovirus-infected cells. However, NK cells,
CD8+ (Tc) cells, kill their target cells without need for
recognition of antigen in association with MHC
molecules. NK cells that
have been activated by
secretions from CD4+ T cells will kill their tumor or viral-infected
targets more effectively.
Macrophages Macrophages are
important in the
regulation of immune responses. Besides their role in phagocytosis,
they may function as antigen-presenting cells (APCs) because they
foreign materials and present these antigens to other cells of
the immune system such as T-cells and B-cells. This is one of the
important first steps in the initiation of an immunological response.
Macrophages, stimulated by certain lymphokines, exhibit increased
levels of phagocytosis and are
also secrete cytokines that modulate immune responses.
Dendritic cells Dendritic
cells also originate
in the bone marrow and function as antigen presenting cells (APCs). In
fact, the dendritic cells are more efficient APCs than macrophages.
These cells are usually found in structural compartments of the
lymphoid organs such as the thymus, lymph nodes and spleen. However,
they are also found in the bloodstream and other tissues of the body.
It is believed that they capture and process antigens in lymphoid
organs where an immunological response is initiated. Of particular
is the recent finding that dendritic cells bind large numbers of HIV
may be a reservoir of virus that is transmitted to CD4+ T cells.