Lymphoid System - consists of all of the tissue aggregates and organs composed of lymphoid tissue which function together to produce our specific resistance to disease (immunity).

Lymphoid Tissue - consists primarily of two tissue components:

1. Reticular tissue which forms the structural matrix supporting the functional tissue. It is chiefly reticular connective    tissue.
2. Lymphatic tissue which is composed of lymphocytes and macrophages.

    These two components will form a diffuse, simple aggregate in many tissues of the body, for example:

1. An infiltration of the lamina propria of mucous membranes in the alimentary canal (Peyer's patches) and respiratory tract (tonsils).

2. In regions subjected to chronic inflammatory reactions.

3. In loose connective tissue.

4. In the walls of viscera.

   When these two tissues (lymphatic and reticular) are the predominant components in an organ, it is referred to as a lymphoid organ. Examples of lymphoid organs would be the lymph nodes, spleen and thymus.

The Lymph Node

1. An oval structure, 1 to 25mm in diameter.

2. Enclosed by a capsule with an internal framework of trabeculae consisting of collagenous and reticular fibers.

3.  Found primarily in the proximal area of the limbs, i.e., axilla, inguinal and cervical nodes, as well as, the retroperitoneal area of the pelvis and abdomen and the surface of thoracic and abdominal organs.

4.  Contains two inner regions:

a.  a cortex consisting of densely packed nodules of lymphocytes found just below the subscapular sinus. The nodules are called primary nodules.  Each one contains lymphocytes, plasma cells and macrophages. During periods of infection or inflammation, the lymphocytes in the nodules divide forming the very active "germinal centers"

b.  a medulla containing cords or rows of lymphocytes separated from one another by sinuses. There are no germinal centers here.

5.  Path of lymph through the node:

a.  afferent lymphatic empties lymph into the subcapsular sinus.

b.  lymph flows through sinuses around germinal centers, along side trabeculae and through the medulla. Cells of the RES line these sinuses and will phagocytize foreign material.

c.  lymph leaves the node through the efferent lymphatic.

d.  lymphocytes move readily with the lymph. The B and T lymphocytes can leave the circulating blood through venule walls and enter lymphatic sinuses and germinal centers. Lymphocytes can also pass into the blood from lymph sinuses or travel with the lymph for reentry via the right lymphatic and thoracic ducts.

          e. sinusoids of the node are lymphatic not blood vascular.

The Spleen - located in the upper left abdominal quadrant.  Unlike the lymph node, the spleen is inserted in the blood stream. The spleen clears the blood of aged blood cells and foreign material.  It is the site of an immune response to blood-borne antigens, especially in children.  In adults, the spleen is not essential to life.  Histologically, its structure resembles the lymph node except:

1. Only efferent lymphatics drain the spleen.

2. The sinusoids are blood vascular not lymphatic.  As a result, all cells entering the spleen do so via the blood.

3. Scattered primary nodules (germinal centers) are found throughout the spleen. These contain abundant lymphocytes and make up the "white pulp" (also known as Malpighian corpuscles). Plasma cells and macrophages are found here as well.

4. Surrounding the white pulp nodules are venous sinuses containing abundant erythrocytes - the "red pulp". These sinuses are lined with macrophages(RE cells) which are highly phagocytic. Between the sinuses are cords of cells - lymphocytes, monocytes and macrophages - referred to as the Cords of Billroth.

5. The immune function of the spleen  is especially important in infancy and early childhood. At these times, the rest of the lymphoid system is somewhat underdeveloped.

         6. In general:

a. white pulp is the site of antibody synthesis and lymphocyte production.

b.  Red pulp is chiefly concerned with the removal and destruction of worn out erythrocytes.

The Thymus - a bilobed mass of lymphoid tissue located in the anterior mediastinum, deep to the manubrium. A large gland at birth, it continues to grow until puberty (35 grams). It then begins to decrease in size and function (12 to 14 grams at 60 years of age).

1. Each lobe is subdivided by collagenous septa into lobules.

2. Each lobule consists of:

a. a peripheral cortex consisting of lymphocytes.  The blood vessels in the cortex are surrounded by tissue - actually reticular cells which form an interconnected network surrounding the capillaries in the cortex. This "blood - thymic barrier" prevents macromolecular substances from leaving the blood. This isolates the thymic lymphocytes from antigenic or immunogenic materials in the blood. However, lymphocytes are able to cross this barrier and enter the circulation in the medulla.

b. a medulla lacking lymphocytes but containing glandular tissue.
                                                                                Lymphocytes from the cortex enter the blood here.

3. Various thymic hormones produced by the medulla regulate the differentiation of T lymphocytes, for example, thymosin and thymopoietin.

4. Decreased lymphatic infiltration, i.e., no afferent vessels or lymph sinuses penetrate the lobule. Lymphatic vessels travel through the interlobular connective tissue.

5. Role of T lymphocytes is critical and essential for a functional immune response.

Mucosa-Associated Lymphoid Tissues

    Lymphoid tissue located beneath the mucosal epithelial lining of the respiratory and digestive systems protects the body against pathogens that may enter the body via the mucosa.

The Tonsils- The tonsils are accumulations of lymphoid tissue surrounding the openings of the digestive and respiratory tracts.

A.  Palatine tonsils - are located in the lateral wall of the oropharynx and covered by a stratified squamous epithelium.

B. Lingual tonsils - are situated in the lamina propria at the root of the tongue and also covered by a stratified squamous epithelium.

C.  Pharyngeal tonsils - are located in the upper posterior part of the throat and covered by a pseudostratified ciliated epithelium with goblet cells.

    The tonsils do not have afferent lymph vessels.  Efferent lymph vessels are present.  Exposure to antigens relies on the contact of antigens with cells of the immune system across the epithelium which covers the tonsils.  The epithelium of the palatine and lingual tonsils forms deep crypts into the lymphoid tissue, and the resulting increase of the surface area is one way to facilitate the contact of antigens with the immune cells.

Peyer's Patches - Small accumulations of lymphocytes or solitary lymph follicles are found scattered beneath the mucosa throughout the gastrointestinal tract.  The most prominent accumulations occur in the ileum and appendix in the form of Peyer's patches.  In the ileum, they form dome-shaped protrusions into the lumen.  Beneath the epithelial lining of the domes, Peyer's patches extend from the lamina propria to the submucosa.  Within Peyer's patches, lymph follicles with germinal centers are typically located deep in; the submucosa.