Lymphatic Tissues and Organs (The Immune System):

When antigens enter the body, two types of immunological responses may occur:

1) the production and release of humoral antibodies into the blood stream and into other body fluids by way of plasma cells. These antibodies act by combining with, and neutralizing bacterial toxins, and by coating bacteria to enhance their chances of being phagocytized.

2) the production of 'sensitized' lymphocytes which have receptors on their surface capable of binding to antigen-presenting cells. These are the cells that effect the cell-mediated immunity that is demonstrated in skin graft or transplant rejection and the 'delayed' hypersensitivity response to tuberculin tests.

On a functional basis then, the lymphoid organs are classified as and secondary lymphoid organs. The thymus and the bursa equivalent in man are the primary ones and all others are secondary.

On a morphological basis, the lymphoid organs can be classified as 1) diffuse, 2) dense, non-encapsulated or 3) dense nodular encapsulated.

Lymphocytes are present and are produced in the lymphatic tissues. Monocytes too may be produced here. The lymphatic tissues and organs have a framework (except for some diffuse types) of reticular fibers and reticular cells. The interstices of this framework are filled with lymphocytes of various sizes and with plasma cells.

A) Diffuse Lymphatic Tissue - diffuse lymphatic tissue is found under wet epithelial surfaces (within the connective tissue of the lamina propria). In Slide #4 of the esophagus, diffuse aggregates of lymphocytes are seen just beneath the covering eplithelium.

What type of epithelium is this? What size lymphocytes can you find here? Many lymphocytes in a section give it ________ color. Why?

B) Dense, Non-encapsulated Lymphatic Tissue - Slide #34 of palatine tonsil demonstrates lymphatic tissue that is organized into small balls or nodules, which may or may not have germinal centers. The areas between nodules may also be dense but not well organized. Note the lympocyte infiltration into the overlying epithelium.

What is the significance of a germinal center?

The stroma (connective tissue framework) is composed of reticular fibers with associated large reticular cells. These may not be readily visible.

C) Dense, Encapsulated Lymphatic Tissue -

i) Lymph Node - Examine Slides #36, 133. The lymph nodes are ovoid or bean-shaped bodies formed at intervals along the course of lymph vessels. Many afferent lymphatics enter the convex surface, while a few efferent vessels leave at the hilus (concave surface).

Note the connective tissue capsule surrounding the node. Fat and loose connective tissue is likely to be adherent to the capsule. From the capsule, connective tissue trabeculae extend into the node. The cortex of the node (outer zone) contains abundant lymphatic tissue and nodules; the medulla is less dense containing cords of lymphatic tissue (medullary cords). Between the nodes and the capsule, and the nodes and the trabeculae, loose lymphatic tissue forms the subcapsular and cortical sinus respectively, and between the medullary cords, the medullary sinuses.

Examine the subcapsular sinus. You will find that it contains lymphocytes as well as large cells with abundant, granular cytoplasm; these are macrophages. In addition to macrophages and lymphocytes, the subcapsular sinus contains antigen-presenting cells - the dendritic reticular cells. These are pale-staining cells with long thin cytoplasmic processes. The outer (capsular) wall of the sinus is lined with a continuous layer of flattened endothelial cells, as are the thin connective tissue strands which crisscross the sinus. The inner border of the sinus has a discontinuous lining of endothelial cells.

Can you identify these?

Next, examine the cortical area of the node. This region contains abundant lymphatic tissue in the form of nodules.

Do these have lighter-staining germinal centers?

These nodules have tails that continue into the medulla to form the medullary cords. That region of the node between the base of the germinal center (if it exists) and the medullary cords, is called the deep cortex or thymus-dependent zone. Do you understand the functional significance of this thymus-dependent zone? Within this zone are found special blood vessels (postcapillary venules), the lining cells of which are cuboidal and quite different from the usual flat endothelial cells that line most other vessels.

Can you find a postcapilllary venule?

H&E stain does not differentiate between "B" and "T" lymphocytes. You should by now know that B-lymphocytes are found _________, while T-lymphocytes are located___________ .

Are the lymphocytes of the nodules large or small? Where are most of the small lymphocytes located? Can you find plasma cells? - Where? The large cells with pale and indistinct cytoplasm that are often seen in the nodules, are the dendritic reticular cells that are capable of binding antigen on their surface.

ii) Spleen - Examine Slides #55, and 119. First, study the topography of the organ under low power. Localize the capsule and the trabeculae, then examine the splenic pulp. Differentiate between white pulp and red pulp.

The key to the understanding of splenic structure lies in the comprehension of the splenic circulation pattern. The arteries (muscular arteries) and the veins, travel within the trabeculae. The veins have much larger lumens. On leaving the trabeculae, the arteries become surrounded by the white pulp as the periarterial sheath (PALs). In fact, the white pulp forms the tunica adventitia of these vessels. Periodically along the course of these arteries, the white pulp widens out to form lymphatic nodules (splenic follicles). The artery seen within the white pulp is called the 'central artery' although it is usually away from the center (eccentric). On leaving the white pulp, the arteries enter the red pulp and divide into several, small, straight vessels, and they are called penicillar arteries, which in turn form the terminal arterial capillaries. These capillaries connect to the venous sinuses which in turn coalesce into the pulp veins and finally into the veins of the trabeculae.

Do you understand the various theories concerning the splenic circulation?

Under higher magnification, can you identify the various cells of the white pulp (small and larger lymphocytes; plasma cells)? Remember that immunological studies have shown that T-lymphocytes populate the periarterial sheath, while B-lymphocytes are found in the nodules and in the adjacent marginal zone. The marginal zone is also important, since cells here (dendritic reticular cells) trap antigen.

The red pulp is composed of venous sinuses and cellular cords (splenic cords) which may be compressed in ordinary histological preparations. Only the general outline of the red pulp and some of the component cells can be studied in most of these sections; Slide #55 is the best.

Slide #60, a section of the spleen stained specifically for reticular fibers, demonstrates the extensive reticular network that forms the framework for this organ. Besides this reticular fiber network; the reticular cells with their processes form a cellular network.

What is the shape and the arrangement of these cells in the spleen?

iii) Thymus - The thymus is a bilobed organ situated just behind the sternum. This gland is composed essentially of epithelial cells and large numbers of lymphocytes, many of which are actively dividing. The occurrence of frequent thymic abnormalities in children with immunological deficiency disorders led to the suggestion that the thymus related in some way to the development of immune responses. The relationship has been classified and it can now be demonstrated that the removal of the thymus gland in mice at birth leads to:

a) decrease in circulating lymphocytes;

b) severe impairment of graft rejection;

c) reduced humoral antibody response to some but not all antigens;

d) wasting after 1-3 months, probably a result of inability to combat infection effectively since neonatally thymectomized mice reared under germ-free conditions did not waste.

The defects caused by neonatal thymectomy can be partially reversed by soluble thymus extracts; a hormone, "thymosin" has been postulated. X-irradiation of adult mice destroys the ability of their lymphocytes to divide and hence their immunological responsiveness. This can be restored by injection of bone marrow cells. However, bone marrow cells fail to restore X-irradiated adult mice which have been thymectomized; on the other hand, adult spleen and lymph node cells were effective. It is thus concluded that the thymus acts on primitive cells coming from the bone marrow to make them immunologically competent ("T" cells).

Examine Slides #42 (fetal); #45 (infant) and #54 (adult). A thin connective tissue capsule surrounds each of the two lobes. The capsule sends septa into the substance of the organ and the lobes are divided into lobules. The lobules can easily be seen under the low power. The peripheral, darkly stained portion is the cortex; the paler central zone is the medulla. The division into lobules is not complete so that the medullary regions of adjacent lobules are continuous at certain points. This medullary continuity can be seen in some parts of the sections. Note that there are no lymphatic nodules in the thymus.

In the other lymphatic organs examined, reticular fibers and reticular cells form the framework. In the case of the thymus, reticular fibers are largely missing and reticular cells, which are derived from connective tissue (mesenchyme) are not present. Instead, stellate cells of endodermal origin (epithelial-reticular cells) constitute the framework. The framework is filled mainly with lymphocytes of various sizes. Thymic corpuscles (Hassal's bodies) are prominent in the medulla of both slides.

Now compare the adult thymus with that of the fetus and infant.

Can you find any difference with regard to lymphocyte population density and the amount of adipose tissue? What is involution?

iv) The Bursa of Fabricius - In birds, another lymphoid organ termed the Bursa of Fabricius is recognized. It is similar to the thymus and is derived embryologically from gut epithelium too. Just as the thymus appears to act as a central lymphoid organ, controlling the maturation of lymphocytes concerned largely with cell-mediated immunity ("T" cells), so the Bursa is responsible for the development of immunocompetence in cells destined to make humoral antibody ("B" cells).

Thus there appears to be two different populations of small lymphocytes:

a) T-lymphocytes, processed by or in some way dependent on the thymus, and responsible for cell-mediated immunity.

b) B-lymphocytes, which are bursa-dependent, and concerned in the synthesis of circulating antibodies.

Both populations proliferate and undergo morphological changes when appropriately stimulated by antigens. The B-lymphocytes develop into the plasma cell series which are active in the synthesis and secretion of antibodies (humoral). T-lymphocytes transform into lymphoblasts which are concerned with the synthesis of their own components and of antibodies which are built into the plasma membrane; they do not secrete appreciable amounts of free antibodies.

The equivalent of the bursa in man and in other mammals has not yet been clearly defined, although gut-associated lymphoid tissues such as the tonsil, appendix and Peyer's patches have been nominated as possible candidates.