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The Immune System The Immune SystemIt is a specialized form of connective tissue that consists of groups of cells, tissues and organs distributed throughout the body.Their function is to protect the body from the invasion and damage by microorganisms and foreign materials by destroying or neutralizing them. This system includes:Spleen ,thymus,lymph nodes,tonsils, Peyers patches, lymphatic(lymphoid) nodules and diffuse lymphatic tissue. The cells, tissues and organs of the immune system are supported by reticular fibers(collagen III) formed by reticular cells.

The cells of the immune system : Lymphocytes,macrophages, plasma cells, granulocytes and antigen presenting cells.These cells communicate with each other and with cells of other systems through signaling proteins called cytokines. The cells of the immune have the ability to distinguish self ( the individuals own macromolecules) from nonself (foreign substances).

In early fetal life, the precursor cells which give rise to stem lymphoctes, originate in the yolk sac, liver and spleen.By late fetal life and the postnatal period the precursor cells are restricted to the bone marrow. The undifferentiated stem lymphocyes migrate to sites in the bone marrow(B lymphocytes) or to the thymus(T lymphocytes) where the undergo differentiation into immunocompetent cells. Therefore the bone marrow and thymus are primary organs. From the primary organ T&B lymphocytes then migrate to the rest of the lymphatic organs and tissues where they undergo antigen- dependent differentiation and proliferation into effector and memory. These organs are called secondary organs. Classification: 1. Primary(Central) e.g Thymus, Bone MarrowSecondary(Peripheral) e.g Spleen , Lymph Nodes , Tonsils, Lymphatic nodules, Diffuse lymphatic tissues 2. Encapsulated or Unencapsulated. The encapsulated organs are spleen, lymph nodes and thymus. The rest are unencapsulated. The thymus is a primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes.The thymus is enclosed by a thin c.t. capsule from which numerous septa extend into the thymus subdividing the 2 lobes into numerous lobules. Each lobule is divided into:Cortex : darker peripheral zone because lymphocytes are more abundant in the cortex. Medulla lighter central zone because epithelial reticular cells are more abundant in the medulla.

Epithelial reticular cells in the medulla form Hassal's corpuscles. Their cytoplasmic extensions form support for parenchyma and blood vessels forming the blood- thymus barrier. Macrophages are also present in addition to the lymphocytes and epithelial reticular cells.

Lymph NodeIt is an encapsulated secondary lymphoid organ.The capsule also sends trabeculae which divides thelymph node into lobules. The lymph node isdivided into: Parenchyma

Stroma ParenchymaIt is the functioning part of any organ. Thisparenchyma is further divided into :

1.Cortex

2.Deep Cortical Region (Para Cortical Region)

3.Medulla

The lymph node is a filter for lymph that is why it ispresent in the lymphatic vessels. The afferent fiberswill enter from the cortex while the efferent fibersleave from the medulla.

Cortex

It is found just under the capsule. It consists oflymphoid follicles or lymphoid nodules which areconcentric structures of lymphocytes. The follicles\are of two types:

Primary Follicles: formed by lymphocytes that have not been activated. They are activated when the antigen comes in contact with it. Secondary Follicles: formed by lymphocytes that have been activated after coming in contact with an antigen. It has a germinal center which is pale because the cells in the center start to divide and their chromatin is dispersed.

.Para Cortical Region

It consists of diffused lymphocytes which are Tlymphocytes that is why the Para Cortical Region isknown as a T-dependant zone. In case ofthymomectomy at infancy the Para Cortical Regionwill not develop. MedullaIt is formed of medullary cords between which liethe medullary sinuses which are dilated capillarieslined by endothelium and are full of lymph. Themedullary chords are formed of B & T lymphocytes,macrophages & other WBCs.

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The Medulla

A Medullary Sinus STROMA It is the connective tissue part of any organ, supporting the parenchyma and the sinuses. It consists of:1.Capsule

2.Trabeculae

3.Reticular Fibers

.SYSTEM OF SINUSESThese are found between the medullary chords and are divided into:

1.Sub Capsular Sinus which is a single large sinus under the capsule. It receives lymph from afferent lymphatic vessels.2.Peri Trabecular Sinuses which are multiple smaller sinuses found surrounding the trabeculae. They receive lymph from the Sub Capsular Sinuses.3.Cortical Sinuses are found in the Para Cortical Region. They receive lymph from the Peri Trabecular Sinuses and drain into the Medullary Sinuses.

4.Medullary Sinuses : are found in the medulla and they drain their lymph into the efferent lymphatic vessels.The system of sinuses is supported byreticular fibers and it increases the divisionof vessels which slows down lymph flow andincreases the number of cells in contact withthe lymph. Both these factors enhance theability of the lymph nodes to filter thelymph.

A Medullary Sinus

SpleenThe spleen is a filter for blood. It clears the blood of aged blood cells and foreign particles and is the site of immune reactions to blood-borne antigens.

It is an encapsulated lymphoid organ which alsoconsists of stroma and parenchyma.

The parenchyma of the spleen consists of:

1.White Pulp: lymphoid follicles or nodules which can also be primary or secondary follicles. This is different from the follicles of the lymph nodes due to the presence of a central artery.

2.Red Pulp: surrounds the white pulp and it is also divided to splenic cords and splenic sinuses.Splenic Cords: Known as Splenic Cords of Bilroth & they consist of all WBCs, macrophages, RBCs and B & T Lymphocytes.Splenic Sinuses: full of blood because the spleen acts as a filter of blood. Splenic sinuses are lined by endothelium.

The stroma of the spleen is similar to that of the lymph node with a capsule, trabeculae & reticular fibers

The Spleen

The Spleen

Spleen macrophages in active phagocytosis of erythrocytes The splenic artery reaches the spleen through the trabeculae by trabecular arteries, which continue as red pulp arteries that enter the white pulp as central arteries.

The WBCs surrounding the central artery form a sheath which is known as the peri-arterial lymphatic sheath . This is also a T-Dependant Zone which may not develop in case of thymomectomy. The central artery leaves the white pulp to become the penicilar artery (straight artery), which open into splenic sinuses that drain into the splenic veins.

Diagram showing how the blood enters and leaves the spleenTonsils 1.Palatine Tonsils: between oral cavity proper and oral pharynx. (Present in the region where air enters & food passes therefore they are often inflamed).

They are uncapsulated but they have a hemi-capsule which separates the lymphoid tissue from adjacent structures. It consists of bands of dense connective tissue. This capsule acts as a barrier against the spread of tonsiliar infections.

These tonsils are covered by non-keratinized stratified squamous epithelium.The most important feature in the palatine tonsils is the presence of crypts (it is in these crypts that the food is impacted. Putrification of this food leads to inflammation)

Under the epithelium are the follicles or nodules which are of two types primary and secondary. The rest of the parenchyma is formed of scattered lymphoid tissue.

2.Nasopharahgeal: represents aggregates or collections of lymphatic nodules found in the nasal cavity. They are covered by psuedostratified columnar ciliated epithelium. Formed of diffused lymphatic tissue and lymphatic nodules.Hypertrophy of these tonsils is called adenoids. (When these tonsils are inflamed they block the nose).3.Lingual: collection of lymphoid follicles that are scattered in the tongue. It is covered by non-keratinized stratified squamous epithelium. Each has a small single crypt.

Mucosa-Associated Lymphoid Tissue: It consists of:1.Gut-associated lymphoid tissue (GALT) is found throughout the gastrointestinal tract. However, the most prominent accumulations occur in the ileum in the form of Peyer's patches and in the appendix. 2. Bronchus- associated lymphoid tissue(BALT)