2nd year medical technology.
Laila Hamed Damanhouri
Peripheral and Secondary lymphoid organs
Objective.
1. Explain how secondary lymphoid organs are organized and how this impacts the
immune response.
2. Explain why all secondary lymphoid organs contain follicles but other structures differ
depending on the organ.
3. Describe the unique features of lymph nodes, spleens, and mucosal associated
lymphoid tissues.
Introduction.
Secondary lymphoid organs are the sites where acquired immune responses are
initiated and are scattered throughout the body, but they are concentrated in areas
that are most likely to be invaded by pathogens (i.e. the gastrointestinal tract, the
respiratory tract, & the genitourinary tract). Among the secondary lymphoid
organs are lymph nodes, spleen, and mucosal-associated lymphoid tissues (MALT),
which includes gut-associated lymphoid tissues (GALT), and bronchial-associated
lymphoid tissues (BALT).
The secondary lymphoid organs supports the function of cells of the lymphoid lineage by
facilitate the encounter of these cells with antigens; they are interconnected by two
circulatory systems; the blood and the lymph circulatory systems. Immune cells travel
through the body via these two circulatory systems. Foreign antigens can also circulate
through these systems, passing through secondary lymphoid organ and interact with
lymphoid cells.
Spleen.
Location. The spleen lies at the upper left quadrant of the abdomen, behind the stomach.
Structure.
It is formed from outer layer consists of capsule of collagen fibers, and two types tissues
the white pulp and red pulp.
The red pulp primarily contains erythrocytes, including numerous dying red blood cells
as well as macrophages that contain engulfed red blood cells.
The white pulp can be divided into:Central artery that surrounded by lymphoid tissues and forming periarterial lymphatic
sheath (PALS).
A. PALS are T- lymphocytes rich area and also contain dendritic cells that act as antigen
presenting cells.
B. The marginal zone, located peripherally to PALS, is rich in B cells which organized
into;
1) Primary (unstimulated follicle) which formed mainly from virgin B cells, 2) secondary
(stimulated follicles) forming a germinal center. The germinal center contain follicular
dendritic cells and macrophages,
Blood entering through splenich artery to enter marginal zone. B cells stimulated by
antigens migrate to PALS to collaboration with T cells that reside in PALS. If B-cell-T
cell collaboration is successful, activated B and T cells migrate into primary follicle,
which become a secondary follicle.
Function of spleen.
1. It is the site of filtration of blood for pathogens and old or damaged blood cells.
This filtration function occurs in the red pulp of spleen.
2. The white pulp of the spleen; comprise an important immunologic organ.
Lymph nodes.
Lymph nodes are small bean shaped structures lying along the course of lymphatics.
They are aggregated in particular sites such as the neck, axillae, groins and paraaortic region. Lymph nodes are scattered throughout the body and filter out microbes
or damaged tissue. Lymph nodes are responsible for the acquired immune response
against antigens encountered in the skin or connective tissues.
Function of Lymph nodes.
LN has two main functions:
1. phagocytic cells act as filters for particulate matter and micro-organisms
2. Antigen is presented to the immune system.
Structure of lymph nodes.
Lymph node is primarily consists of three components.
1. lymphatic sinuses. Afferent lymphatics deliver fluid from the tissues to the
subcapsular sinus of the lymph node. This fluid contains cells and antigens that
stimulate the acquired immune system.
2. Parenchyma. Parenchyma is divided into three important areas (cortex, paracortex,
medulla.). B cells are found predominantly in the outer cortex pf lymph nodes which
contain a number of dense aggregations of lymphocytes termed follicles. These
enlarge during an active immune response to form germinal centers which contain
large number of proliferating B lymphocytes surrounded by a mantle zone consisting
of small, naive B cells and a few T cells.
Paracortex. The paracortex contains lymphocytes and accessory cells along with
supporting cells and it is the predominant site for T lymphocytes within the lymph
node. In addition there are numerous Interdigitating cells in the paracortex and they
act as antigen presenting cells.
Medulla:- The medulla comprises of large blood vessels, medullary sinuses and
The medullary cords which are rich in plasma cells which produce antibodies that
pass out of the node via the efferent lymphatic. Macrophages are also numerous
within the medulla.
Mucosa-associated lymphoid tissue (MALT)
In addition to the lymphoid tissue concentrated within the lymph nodes and spleen,
lymphoid tissue is also found at other sites, most notably the gastrointestinal tract,
respiratory tract and urogenital tract.
Gut associated lymphoid tissue (GALT)
This comprises: tonsils, Peyer's patches in intestine, lymphoid aggregates in the appendix
and large intestine
Peyer's Patches
These are quite large aggregates of lymphoid tissue found in the small intestine. These
lymphoid tissues are covered with specialized epithelium contain specialized cells and
known as M cells. They and are believed that these cells an important in the transfer of
antigen from the gut lumen to Peyer's Patches.
Peyer's Patches facilitate the generation of an immune response within the mucosa. B cell
precursors and memory cells are stimulated by antigen in Peyer's Patches.
6) Lymphocyte recirculation
Lymphocytes and some mononuclear phagocytes can recirculate between lymphoid and
non-lymphoid tissues. This helps in allowing lymphocytes to be exposed to the antigens
which they recognize
1. virgin lymphocytes move from the primary to secondary lymphoid tissue via the
blood . 2. activated lymphocytes move from the spleen, lymph nodes and MALT into
the blood and thene to other lymphoid and non-lymphoid tissues. 3. antigen
presenting cells such as macrophages and dendritic cells may carry antigen back to
lymphoid tissues from the periphery.