THE LYMPHATIC SYSTEM IS COMPOSED OF TWO SEMI-INDEPENDENT PARTS
1. Various lymphoid tissues and organs scattered throughout the body
2. An extensive network of lymphatic vessels
1. LYMPHOID TISSUES AND ORGANS SCATTERED THROUGHOUT THE BODY
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LYMPH NODES: thousands are located throughout the body. They are responsible for removing
foreign material such as bacteria, viruses, cell debris, proteins, etc, from the plasma (clear
water) as it returns to the blood capillaries for re-circulation in the body.

SPLEEN: located on the left side of the abdominal cavity and extends to curl around the anterior
aspect of the stomach. It is a blood rich organ that filters blood. The spleen filters and cleans
blood of bacteria, viruses and other debris. It also stores platelets, acting as a blood reservoir.
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THYMUS: found low in the throat overlying the heart. Functions at peak levels in youth as it
becomes congested with fat, and less active in older persons. It produces hormones (thymosin
and others) that function in the programming of certain lymphocytes so they can carry out their
protective role in the body.
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MUCOSA ASSOCIATED LYMPHATIC TISSUE (MALT): is the diffuse system of small concentrations
of lymphoid tissue found in various sites of the body such as the gastrointestinal tract, thyroid,
breast, lung, salivary glands, eye, and skin. MALT is populated by lymphocytes such as T cells and
B cells, as well as plasma cells and macrophages, each of which is well situated to encounter
antigens passing through the mucosal epithelium.

TONSILS: small masses of lymphatic tissue that are found in the mucosa and ring the pharynx
(the throat). Their job is to trap and remove bacteria or other foreign pathogens entering the
throat.

PEYER’S PATCHES: resembles tonsils and are located in the wall of the small intestine. The
macrophages of the Peyer’s patches capture and destroy bacteria (always present in
tremendous numbers in the intestine) thereby preventing bacteria from penetrating the
intestinal wall.
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APPENDIX: now considered a part of our immune system as well as stimulating the movement
of material from the small intestine through the ileocecal valve and cecum into the large
intestine to begin its movement up the ascending colon.
2. AN EXTENSIVE NETWORK OF LYMPHATIC VESSELS

LYMPHATIC VESSELS / CAPILLARIES: The smallest lymphatic vessels are the lymph
capillaries, which begin in the tissue spaces as blind-ended sacs. Lymph capillaries are found in
all regions of the body except the bone, marrow, central nervous system, and tissues such as the
epidermis that lack blood vessels.
Lymphatic Capillaries
a) Microscopic in size.
b) One cell layer of simple squamous epithelium.
c) Poor "fit" between adjacent cells results in porous walls.
d) In intestinal walls they are called lacteals (for fat absorption).‫‏‬
Initial lymphatic
a) Drawing of the different vessel layers (endothelium and extracellular matrix with anchor
filaments)
b) Scanning electron microscopy showing the extracellular matrix of an initial lymph vessel
after removal of the endothelial layer in the tongue tissue of a rat.
a)
b)
Courtesy of Prof. Dr. A. Castenholz Kassel, with permission from Ch.L. Witte, Editor in Chief, LYMPHOLOGY)

LYMPHATIC COLLECTORS / LYMPHANGION: The functional unit of a lymph vessel
that lies between two semilunar (half-moon shaped) valves. Lymph vessels are channels larger than the
lymph capillaries that have thicker walls, valves in their lumen and smooth muscles in their walls, thus
lymph vessel lymphangion are muscular and capable of contracting on its own. Additionally, the lymph
in it is propelled forward only because of force exerted on its walls from the exterior. Such forces
include skeletal muscle contractions and arterial pulsations. Also, the inspiration during respiration
provides a suction pressure within the lumen.
The semilunar valves are directedtowards the flow of the lymph and open when the pressure in the first
lymphangion is greater than the pressure in the next lymphangion. Pressure in the first lymphangion
may increase because of smooth muscle contraction (in lymph vessel) or because of pressure on the
walls from outside (in a capillary). Alternatively, pressure within the next lymphangion may decrease
because of negative pressure as a result of inspiration. Once the lymph flows into the next lymphangion,
it cannot return to the previous lymphangion because the semilunar valves close tightly.

LYMPH NODES: There are thousands located throughout the body. They are responsible for
removing foreign material such as bacteria, viruses, cell debris, proteins, etc. from the plasma (clear
water) as it returns to the blood capillaries for re-circulation in the body. They range from pinhead size
to lima bean size. They are found singly or in clusters. They are located by body openings or where
there are appendages, and the biological filtration by phagocytosis of bacteria and other abnormal cells
prevents infections from spreading.
Production of lymphocytes is increased when the flow of lymph is increased through the lymph nodes.
And, lymph nodes can become blocked / clogged with debris and result in severe inflammation.