Lymphoid System

• Diffuse Lymphoid Tissue– GALT– BALT

• Lymph node

• Spleen

Diffuse Lymphoid Tissue.• Diffuse lymphoid tissue is especially prominent

in the mucosa of the gastrointestinal and respiratory systems.

• It is organized as nonencapsulated clusters of lymphoid cells or as lymphoid (lymphatic) nodules.

• Diffuse lymphoid tissue is collectively called mucosa associated lymphoid tissue (MALT). – MALT consists of two major types, bronchus-

associated lymphoid tissue (BALT) and gut-associated lymphoid tissue (GALT). Both types possess lymphoid nodules that are isolated from one another, except in the case of Peyer patches.

– Peyer patches are aggregates of lymphoid nodules found in the ileum. They are components of the GALT.

• Lymphoid (lymphatic) nodules are transitory, dense, spherical accumulations of lymphocytes (mostly B cells).

• The dark, peripheral region of nodules (corona) is composed mainly of small, newly formed lymphocytes.

• Lymphoid nodules of the GALT are isolated from the lumina of their respective tracts by microfold (M) cells, which transfer antigens from the lumen and present them (without processing them into epitopes) to lymphocytes and macrophages lying in deep invaginations of their basal cell surfaces.

• Primary nodule.– The nodule present before antigen stimulation, it consists primarily of

densely packed spheres of B lymphocytes.• Secondary nodule.

– After antigen stimulation, a central pale core,the germinal center, appears.

– This center is composed of immunoblasts that divide to form lymphocytes that accumulatein the densely packed, peripheral zone.

Tonsils • are aggregates of lymphoid tissue, which sometimes lack a capsule. • All tonsils are located in the upper section of the digestive tract,

lying beneath, but in contact with, the epithelium. Tonsils assist in combating antigens entering via the nasal and oral epithelia.

• Palatine tonsils: – Possess crypts, deep invaginations of the stratified squamous

epithelium covering of the tonsils; the crypts frequently contain debris – Possess lymphoid nodules, some (secondary nodules) with germinal

centers – Are separated from subjacent structures by a connective tissue capsule

• 2. The pharyngeal tonsil is a single tonsil located in the posterior wall of the nasopharynx. – It is covered by a pseudostratified ciliated columnar epithelium. – No crypts

• Lingual tonsils – Lingual tonsils are located on the dorsum of the posterior third of the

tongue and are covered by a stratified squamous nonkeratinized epithelium.

– Each lingual tonsil possesses a deep crypt, which frequently contains cellular debris. Ducts of mucous glands often open into the base of these crypts.

Lymph nodes

Overview-lymph nodes • Nodes are small bodies placed at intervals along the lymphatic

vessels, and structured so that the lymph has to pass through them• A lymph node is a small, encapsulated, ovoid to kidney-shaped

organ with a capsule that sends trabeculae into the substance of the node.

• The convex surface of a lymph node receives afferent lymphatic vessels, whereas the concave surface (the hilum) is the site where arterioles enter and efferent lymphatic vessels and venules exit.

• Lymph nodes possess a stroma composed of a supportive framework rich in reticular fibers.

• Function.– Lymph nodes filter lymph, maintain and produce T and B cells, and

possess memory cells (especially T memory cells). – Antigens delivered to lymph nodes by antigen presenting cells are

recognized by T cells, and an immune response is initiated.

Cells of Lymph Node

• Lymphoid cells

• Plasma cells

• Macrophages and other phagocytic antigen processing cells

• Lymphatic and vascular endothelial cells and fibroblasts responsible for lymph node supporting framework.

Structure-lymph nodes.

Lymph nodes are divided into cortical and medullary regions.

Cortex

• The cortex of lymph nodes:

• Lies deep to the capsule from which it is separated by a subcapsular sinus

• Is incompletely subdivided into compartments by connective tissue septa derived from the capsule

• Contains lymphoid nodules, sinuses, and the inner cortex

• Lymphoid nodules are composed mainly of B cells but also of T cells, follicular dendritic cells, macrophages, and reticular cells. They may possess a germinal center.

• sinuses are endothelium-lined spaces that extend along the capsule and trabeculae and are known as subcapsular and cortical sinuses, respectively.

• The inner cortex is located between the cortex and the medulla. It is composed of a non-nodular arrangement of mostly T lymphocytes. The inner cortex is the region where circulating lymphocytes gain access to lymph nodes via postcapillary (high endothelial) venules.

Section of a lymph node showing the cortex and the medulla 1 Capsule;2 lymphoid nodule with germinative center 3

subcapsular sinus; (4) intermediate sinus; (5) medullary cords; (6) medullary sinus; (7) trabecula.

Medulla

• The medulla of a lymph node lies deep to the inner cortex and cortex except at the region of the hilum. It is composed of medullary sinuses and medullary cords.

• Medullary sinuses are endothelium-lined spaces supported by reticular fibers and reticular cells and frequently contain macrophages. Medullary sinuses receive lymph from the cortical sinuses.

• Medullary cords are composed of lymphocytes and plasma cells.

Medulla of Lymph NodeLymphocytes predominate

Medullary sinusesMedullary cords

Spleen

Overview-spleen • A simple squamous epithelium (peritoneum) covers the

capsule of the spleen. The capsule consists of dense irregular collagenous connective tissue, which sends trabeculae into the substance of the spleen to form a supportive framework. The spleen possesses a hilum.

• The spleen differs from the thymus and lymph nodes in that it lacks a cortex and medulla. It additionally differs from lymph nodes by the absence of afferent lymphatics.

• It is divided into red pulp and white pulp; the latter contains lymphoid elements. These two regions are separated from each other by the marginal zone.

Function.

• The spleen filters blood

• stores erythrocytes

• phagocytoses damaged and aged erythrocytes

• a site of proliferation of B and T lymphocytes

• the production of antibodies by plasma cells

• Vascularization of the spleen is derived from the splenic artery, which enters the hilum and gives rise to trabecular arteries.

• Trabecular arteries leave the trabeculae, become invested by a peri arterial lymphatic sheath (PALS), and are known as central arteries.

• Central arteries branch but maintain their lymphatic sheath until they leave the white pulp to form several straight penicillar arteries.

• Penicillar arteries enter the red pulp. They have three regions: pulp arterioles, macrophage-sheathed arterioles, and terminal arterial capillaries. The latter either drain directly into the splenic sinuses (closed circulation) or terminate as open-ended vessels within the splenic cords of the red pulp (open circulation).

• Splenic sinuses are drained by pulp veins, which are tributaries of the trabecular veins; these, in turn, drain into the splenic vein, which exits the spleen at the hilum.

Structure of spleen

• White pulp of the spleen includes all of the organ's lymphoid tissue (diffuse and nodular), such as lymphoid nodules (mostly B cells) and PALS (mostlyT cells) around the central arteries. It also contains macrophages and other APCs.

• The marginal zone of the spleen: – Is a sinusoidal region between the red and white pulps, located

at the periphery of the PALS – Receives blood from capillary loops derived from the central

artery and thus is the first site where blood contacts the splenic parenchyma

– Is richly supplied by avidly phagocytic macrophages and other APCs

– Is the region where circulating T and B lymphocytes enter the spleen before becoming segregated to their specific locations within the organ

• Red pulp of the spleen is composed of an interconnected network of sinuses supported by a loose type of reticular tissue (splenic cords).

• sinuses: – Are lined by long, fusiform endothelial cells separated

by relatively large intercellular spaces – Have a discontinuous basal lamina underlying the

endothelium and circumferentially arranged "ribs" of reticular fibrils

• Splenic cords (cords of Billroth) contain plasma cells, reticular cells, blood cells, and macrophages enmeshed within the spaces of the reticular fiber network. Processes of the macrophages enter the lumina of the sinuses through the spaces between the endothelial cells.