Human_Histology

88 Textbook of Human Histology

A

B

anchoring points (or focal densities). When the muscle contracts these points are drawn closer to each other. This converts an elongated smooth muscle cell in one that is oval (Figs. 8.16A and B).

Distribution

Smooth muscle is seen most typically in the walls of hollow viscera including the stomach, the intestines, the urinary bladder and the uterus.

It is present in the walls of several structures that are in the form of narrow tubes, e.g. arteries, veins, bronchi, ureters, deferent ducts, uterine tubes, and the ducts of several glands.

The muscles that constrict and dilate the pupil are made up of smooth muscle.

Some smooth muscle is present in the orbit (orbitalis); in the upper eyelid (Muller’s muscle); in the prostate; in the skin of the scrotum (Dartos muscle). In the skin delicate bundles of smooth muscle are present in relation to hair follicles. These bundles are called the arrector pili muscles.

Variations in Arrangement of Smooth Muscle

Smooth muscle fibers may be arranged in a variety of ways depending on functional requirements.

In some organs (e.g. the gut) smooth muscle is arranged in the form of two distinct layers: an inner circular and an outer longitudinal. Within each layer the fasciculi lie parallel to each other. Such an arrangement allows peristaltic movements to take place for propulsion of contents along the tube.

In some organs (e.g. the ureter) the arrangement of layers may be reversed, the longitudinal layer being internal to the circular one. In yet other situations there may be three layers: inner and outer longitudinal with a circular layer in between (e.g. the urinary bladder and vas deferens).

In some regions (e.g. urinary bladder, uterus) the smooth muscle is arranged in layers, but the layers are not distinctly demarcated from each other. Even within layers the fasciculi tend to run in various directions and may form a network.

In some tubes (e.g. the bile duct) a thick layer of circular muscle may surround a segment of the tube forming a sphincter. Contraction of the sphincter occludes the tube.

In the skin, and in some other places, smooth muscle occurs in the form of narrow bands.

Innervation

Smooth muscle is innervated by autonomic nerves, both sympathetic and parasympathetic. The two have opposite effects. For example, in the iris, parasympathetic stimulation causes constriction of the pupil, and sympathetic stimulation causes dilatation. It may be noted that sympathetic or parasympathetic nerves may cause contraction of muscle at some sites, and relaxation at other sites.

Blood Vessels and Lymphatics

Blood vessels and lymphatics are present in smooth muscle, but the density of blood vessels is much less than in skeletal muscle (in keeping with much less activity).

Pathological Correlation

40 3 2%4 (5 )6 7566 7

3 8

3 4 0"8 #39

MYOEPITHELIAL CELLS

Apart from muscle myoepithelial cells show the presence of contractile proteins (actin and myosin). Myoepitheliocytes (or myoepithelial cells) are present in close

relation to secretory elements of some glands. They help to squeeze secretions out of secreting elements. Myoepithelial cells may be stellate, forming baskets around acini, or may be fusiform.

Myoepitheliocytes are seen in salivary glands, the mammary glands, and sweat glands. These cells are of ectodermal origin. With the EM they are seen to contain actin and myosin filaments. They can be localized histochemically, because they contain the protein desmin that is specific to muscle. Myoepithelial cells are innervated by autonomic nerves.

Chapter 8 Muscular Tissue 89

Clinical Correlation

% 3" #" #0 " #0 " #

3 " #

:0

Duchenne muscular dystrophy

The lymphoid system includes lymphatic vessels and lymphoid tissue.

LYMPHATIC VESSEL

When circulating blood reaches the capillaries, part of its fluid content passes into the surrounding tissues as tissue fluid. Most of this fluid re-enters the capillaries at their venous ends. Some of it is, however, returned to the circulation through a separate system of lymphatic vessels (usually called lymphatics).

The fluid passing through the lymphatic vessels is called lymph.

The smallest lymphatic (or lymph) vessels are lymphatic capillaries that join together to form larger lymphatic vessels.

The largest lymphatic vessel in the body is the thoracic duct. It drains lymph from the greater part of the body. The thoracic duct ends by joining the left subclavian vein at its junction with the internal jugular vein. On the right side there is the right lymphatic duct that has a similar termination.

LYMPHOID TISSUE

Lymphoid tissue may be broadly classified as:

Diffuse lymphoid tissue

Dense lymphoid tissue

Diffuse Lymphoid Tissue

Diffuse lymphoid tissue consists of diffusely arranged lymphocytes and plasma cells in the mucosa of large intestine, trachea, bronchi, and urinary tract.

Dense Lymphoid Tissue

It consists of an aggregation of lymphocytes arranged in the form of nodules. These nodules are found either as discrete encapsulated organs or in close association to the lining epithelium of the gut. Dense lymphoid tissue can therefore be divided as:

Discrete lymphoid organs: These include thymus, lymph nodes, spleen, and tonsils.

Mucosa-associated lymphoid tissue (MALT): Small numbers of lymphocytes may be present almost anywhere in the body, but significant aggregations are seen in relation to the mucosa of the respiratory, alimentary, and urogenital tracts. These aggregations are referred to as MALT.

Mucosa-associated lymphoid tissue in the respiratory system: In the respiratory system the aggregations are relatively small and are present in the walls of the trachea and large bronchi. The term bron- chial-associated lymphoid tissue (BALT) is applied to these aggregations.

Mucosa-associated lymphoid tissue in the alimentary system: This is also called gut-associated lymphoid tissue (GALT) and includes Peyer’s patches of ileum, adenoids (located in the roof of pharynx), lingual tonsils in posterior 1/3rd of tongue, palatine tonsils and lymphoid nodules in vermiform appendix.

Note: Thymus and bone marrow are primary lymphoid organs while others are secondary lymphoid organs.

LYMPH

Lymph is a transudate from blood and contains the same proteins as in plasma, but in smaller amounts, and in somewhat different proportions. Suspended in lymph there are cells that are chiefly lymphocytes. Most of these lymphocytes are added to lymph as it passes through lymph nodes, but some are derived from tissues drained by the nodes.

Large molecules of fat (chylomicrons) that are absorbed from the intestines enter lymph vessels. After a fatty meal these fat globules may be so numerous that lymph becomes milky (and is then called chyle). Under these conditions the lymph vessels can be seen easily as they pass through the mesentery.

LYMPHOCYTES

The cell population of a lymph node is made up (overwhelmingly) of lymphocytes.

In the embryo lymphocytes are derived from mesenchymal cells present in the wall of the yolk sac, in the liver and in the spleen. These stem cells later migrate to bone marrow. Lymphocytes formed from these stem cells (in bone marrow) enter the blood. Depending on their subsequent behavior they are classified into two types.

Some of them travel in the bloodstream to reach the thymus. Here they divide repeatedly and undergo certain changes. They are now called T-lymphocytes (“T” from thymus). These T-lymphocytes, that have been “processed” in the thymus re-enter the circulation to reach lymphoid tissue in lymph nodes, spleen, tonsils, and intestines.

In lymph nodes T-lymphocytes are found in the diffuse tissue around lymphatic nodules. In the spleen they are found in white pulp. From these masses of lymphoid tissue many lymphocytes pass into lymph vessels, and through them they go back into the circulation. In this way lymphocytes keep passing out of blood into lymphoid tissue (and bone marrow), and back from these into the blood. About 85% of lymphocytes seen in blood are T-lymphocytes (Fig. 9.1).

Lymphocytes of a second group arising from stem cells in bone marrow enter the blood-stream, but do not go to the thymus. They go directly to lymphoid tissues (other than the thymus). Such lymphocytes are called B-lymphocytes (“B” from bursa of Fabricus,

a diverticulum of the cloaca in birds: in birds B-lymp- hocytes are formed here). In contrast to T-lymphocytes that lie in the diffuse lymphoid tissue of the lymph nodes and spleen, B-lymphocytes are seen in lymphatic nodules. The germinal centers are formed by actively dividing B-lymphocytes, while the dark rims of lymphatic nodules are formed by dense aggregations of B-lymphocytes. Like T-lymphocytes, B-lymphocytes also circulate between lymphoid tissues and the bloodstream (Fig. 9.2).

Lymphocytes and the Immune System

Lymphocytes are an essential part of the immune system of the body that is responsible for defense against invasion by bacteria and other organisms. In contrast to granulocytes and monocytes that directly attack invading organisms, lymphocytes help to destroy them by producing substances called antibodies. These are protein molecules that have the ability to recognize a “foreign” protein (i.e. a protein not normally present in the individual). The foreign protein is usually referred to as an antigen.

Every antigen can be neutralized only by a specific antibody. It follows that lymphocytes must be capable of producing a very wide range of antibodies; or rather that there must be a very wide variety of lymphocytes, each variety programed to recognize a specific antigen and to produce antibodies against it.

This function of antibody production is done by B-lymp- hocytes. When stimulated by the presence of antigen the cells enlarge and get converted to plasma cells. The plasma cells produce antibodies.

92 Textbook of Human Histology

Antibodies are also called immunoglobulins. Immunoglobulins are of five main types viz., IgG, IgM, IgA, IgE, and IgD.

T-lymphocytes are also concerned with immune responses, but their role is somewhat different from that of B-lymphocytes. T-lymphocytes specialize in recognizing cells that are foreign to the host body. These may be fungi, virus infected cells, tumor cells, or cells of another individual. T-lymphocytes have surface receptors that recognize specific antigens (there being many varieties of T-lymphocytes each type recognizing a specific antigen). When exposed to a suitable stimulus the T-lymphocytes multiply and form large cells that can destroy abnormal cellsbydirectcontact,orbyproducingcytotoxicsubstances called cytokines or lymphokines (see below). From the above it will be seen that while B-lymphocytes defend the body through blood borne antibodies, T-lymphocytes are responsible for cell mediated immune responses (cellular immunity). T-lymphocytes can also influence the immune responses of B-lymphocytes as well as those of other T-lymphocytes; and also those of non-lymphocytic cells.

Like B-lymphocytes some T-lymphocytes also retain a memory of antigens encountered by them, and they can respond more strongly when the same antigens are encountered again.

Pathological Correlation

The destruction of foreign cells by T-lymphocytes is responsible for the “rejection” of tissues or organs grafted from one person to another. Such rejection is one of the major problems in organ transplantation.

Added Information

Investigations using sophisticated techniques have established that T-lymphocytes can be divided into several categories as follows:

Cytotoxic T-cells (TC cells) attack and destroy other cells by release of lysosomal proteins. They have the ability to recognize proteins that are foreign to the host body.

Delayed type hypersensitivity related T-cells synthesize and release lymphokines when they come in contact with antigens. Lymphokines attract macrophages into the area. They also stimulate macrophages to destroy the antigen. One type of cytokine called interleukin-2 stimulates the proliferation of both B-lymphocytes and T-lymphocytes.

Helper T-cells (or TH cells) play a rather indirect role in stimulating production of antibodies by B-lymphocytes. When a macrophage ingests an antigen some products of antigen break-down pass to the cell surface. Here they combine with special molecules (Class II MHC

Contd...

Contd...

molecules) present in the cell membrane. This complex of antigen remnant and MHC protein, present on the macrophage, can be recognized by helper T-cells. When helper T-cells come in contact with this complex they look for B-lymphocytes capable of producing antibody against the particular antigen. They then stimulate these B-lymphocytes to multiply so that large numbers of B-lymphocytes capable of producing antibody against the particular antigen are produced. This is how immunity against the particular antigen is acquired. Helper T-cells in a loss of immunity.

Suppressor T-cells (or TS cells) have a role opposite to that of helper T-cells. They suppress the activities of B-lymphocytes and of other T-lymphocytes. The possi-! ties of lymphocytes.

Natural killer cells are similar to cytotoxic T-cells, but their" # cells can destroy virus infected cells and some tumor cells. Their structure is somewhat different from that of typical lymphocytes.

Natural killer cells are regarded by some as a third variety of lymphocytes (in addition to B-lymphocytes and T-lymphocytes) as markers in them are different from those in typical T-lymphocytes.

Markers for Cells of Immune System

Many proteins that are specific to cells of the immune system are now recognized. These may be cytosolic or cell membrane proteins. Using antibodies specific to these proteins it is possible to identify many varieties of lymphocytes and macrophages. The proteins are called cluster designation (CD) molecules, and are designated by number (CD-1, CD-2, etc.).

Pathological Correlation

The virus (HIV) that causes the disease called AIDS attaches itself to the protein CD-4 present on the cell membrane, and destroys cells bearing this protein. Reduction in the number of CD-4 bearing lymphocytes is an important indicator of the progress of AIDS.

Cytokines (Lymphokines)

We have seen that T-lymphocytes produce cytokines that affect other cells. The main function of these cytokines is to stimulate production of blood cells and their precursors. Apart from T-cells, cytokines are also produced by monocytes, macrophages, some fibroblasts, and some endothelial cells. Some cytokines that have been identified are as follows:

Interleukins are of eleven known types (IL-1 to IL-11)

Granulocyte macrophage colony stimulating factor (GM-CSF)

Granulocyte colony stimulating factor (G-CSF)

Macrophage colony stimulating factor (M-CSF)

Stem cell factor

Erythropoietin

The cytokines produced by different types of cells are summarized in Table 9.1. The cytokines stimulating production of different blood cells are given in Table 9.2.

MONONUCLEAR PHAGOCYTE SYSTEM

Distributed widely through the body there are a series of cells that share the property of being able to phagocytose unwanted matter including bacteria and dead cells. These cells also play an important role in defense mechanisms, and in carrying out this function they act in close collaboration with lymphocytes. In the past some of the cells of this system have been included under the term reticulo-endothelial system, but this term has now been discarded as it is established that most endothelial cells do not act as macrophages. The term macrophage system has also been used for cells of the system, but with the discovery of a close relationship between these cells and mononuclear leucocytes of blood the term mononuclear phagocyte system (or monocyte phagocyte system) has

come into common usage. It is now known that all macrophages are derived from stem cells in bone marrow that also give origin to mononuclear cells of blood.

Cells of Mononuclear Phagocyte System

The various cells that are usually included in the mononuclear phagocyte system are as follows.

Monocytes of blood, and their precursors in bone marrow (monoblasts, promonocytes).

Macrophage cells (histiocytes) of connective tissue.

Littoral cells (von Kupffer cells) interspersed among cells lining the sinusoids of the liver; and cells in walls of sinusoids in the spleen and lymph nodes.

Microglial cells of the central nervous system.

Macrophages in pleura, peritoneum, alveoli of lungs, spleen, and in synovial joints.

Free macrophages present in pleural, peritoneal and synovial fluids.

Dendritic cells of the epidermis, and similar highly branched cells in lymph nodes, spleen and thymus. These are now grouped as antigen presenting cells (see below).

Structure of Cells

All cells of the mononuclear phagocyte system have some features in common. They are large cells (15–25 μm)

94Textbook of Human Histology

in diameter. The nucleus is euchromatic. Granular and agranular endoplasmic reticulum, Golgi complex and mitochondria are present, as are endocytic vesicles and lysosomes. The cells have irregular surfaces that bear filopodia (irregular microvilli). Most of the cells are more or less oval in shape, but the dendritic cells are highly branched.

Macrophages often form aggregations. In relation to the peritoneum and pleura such aggregations are seen as milky spots; and in the spleen they form ellipsoids around small arteries. When they come in contact with large particles, macrophages may fuse to form multinuclear giant cells (foreign body giant cells). In the presence of organisms like tubercle bacilli the cells may transform to epithelioid cells. (These are involved in T-cell mediated immune responses).

Monocytes can be classified into various types depending on the markers present in them, and their relationship to cytokines (see Tables 9.1 and 9.2).

Origin of Cells

All cells of the system are believed to arise from stem cells in bone marrow (classified as CFU-G/M stem cells). From bone marrow they pass into blood where they are seen as monocytes. From blood they pass into the tissues concerned.

Functional Classification

From a functional point of view mononuclear phagocytes are divided into two main types.

With the exception of dendritic cells all the cell types are classified as highly phagocytic cells.

The dendritic mononuclear phagocytes (now called dendritic antigen presenting cells) are capable of phagocytosis, but their main role is to initiate immune reactions in lymphocytes present in lymph nodes, spleen and thymus (in the manner discussed above). It has been postulated that all dendritic cells are primarily located in the skin. From here they pick up antigens and migrate to lymphoid tissues where they stimulate lymphocytes against these antigens. They are therefore referred to as antigen presenting cells (APCs). Most APCs are derived from monocytes, but some are derived from other sources.

Functions of the Mononuclear Phagocyte System

Participation in Defense Mechanisms

As already stated the cells have the ability to phagocytose particulate matter, dead cells, and organisms. In the lungs,

alveolar macrophages engulf inhaled particles and are seen as dust cells. In the spleen and liver, macrophages destroy aged and damaged erythrocytes.

Role in Immune Responses

All mononuclear phagocytes bear antigens on their surface (class II MHC antigens). Antigens phagocytosed by macrophages are partially digested by lysosomes. Some remnants of these pass to the cell surface where they form complexes with the MHC antigens. This complex has the ability to stimulate T-lymphocytes.

Certain T-lymphocytes produce macrophage activating factors (including interleukin-2) that influence the activity of macrophages. Macrophages when thus stimulated synthesize and secrete cytokines that stimulate the proliferation and maturation of further lymphocytes.

When foreign substances (including organisms) enter the body, antibodies are produced against them (by lymphocytes). These antibodies adhere to the organisms. Macrophages bear receptors (on their surface) that are able to recognize these antibodies. In this way macrophages are able to selectively destroy such matter by phagocytosis, or by release of lysosomal enzymes.

From the above it will be seen that lymphocytes and macrophages constitute an integrated immune system for defense of the body.

When suitably stimulated mononuclear phagocytes secrete a tumor necrosing factor (TNF) which is able to kill some neoplastic cells.

Macrophages influence the growth and differentiation of tissues by producing several growth factors and differentiation factors.

LYMPHATIC VESSELS

LYMPH CAPILLARIES

Lymph capillaries (or lymphatic capillaries) begin blindly in tissues where they form a network. The structure of lymph capillaries is basically similar to that of blood capillaries, but is adapted for much greater permeability. There is an inner lining of endothelium. The basal lamina is absent or poorly developed. Pericytes or connective tissue are not present around the capillary.

As compared to blood capillaries, much larger molecules can pass through the walls of lymph capillaries. These include colloidal material, fat droplets, and particulate matter such as bacteria. It is believed that these substances pass into lymph capillaries through gaps between endothelial cells lining the capillary; or by pinocytosis.

Fig. 9.3: Transverse section across the thoracic duct (Schematic representation)

Chapter 9 Lymphatics and Lymphoid Tissue 95

Contd...

Lymphedema is swelling of soft tissues due to localized increase in the quantity of lymph. It may be primary (idiopathic) or secondary (obstructive). Secondary lymphedema is more common and may be due to lymphatic invasion by malignant lymphedema occurs only when the obstruction is widespread as otherwise collaterals develop. The affected area consists of dilatation of lymphatics distal to obstruction with increased in escape of milky chyle into the peritoneum (chyloperitoneum), into the pleural cavity (chylothorax), into pericardial cavity

(chylopericardium) and into the urinary tract (chyluria).

Lymph capillaries are present in most tissues of the body. They are absent in avascular tissues (e.g. the cornea, hair, nails); in the splenic pulp; and in the bone marrow. It has been held that lymphatics are not present in nervous tissue, but we now know that some vessels are present.

LARGER LYMPH VESSELS

The structure of the thoracic duct and of other larger lymph vessels is similar to that of veins (Fig. 9.3).

A tunica intima, media and adventitia can be distinguished. Elastic fibers are prominent and can be seen in all three layers.

The media, and also the adventitia contain some smooth muscle.

In most vessels, the smooth muscle is arranged circularly, but in the thoracic duct the muscle is predominantly longitudinal.

Numerous valves, similar to those in veins, are present in small as well as large lymphatic vessels. They are more numerous than in veins. The valves often give lymph vessels a beaded appearance.

Clinical Correlation

Lymphangitis may be acute or chronic.

Acute lymphangitis occurs in the course of many bacterial infections. The most common organisms are -hemolytic streptococci and staphylococci. When this happens in vessels of the skin, the vessels are seen as red lines that are painful.

Chronic lymphangitis occurs due to persistent and recurrent acute lymphangitis or from chronic infections like tuberculosis, syphilis and actinomycosis.

Contd...

LYMPH NODES

General Features

Scattered along the course of lymphatic vessels there are numerous small masses of lymphoid tissue called lymph nodes that are usually present in groups.

As a rule lymph from any part of the body passes through one or more lymph nodes before entering the blood stream. (There are some exceptions to this rule. For example, some lymph from the thyroid gland drains directly into the thoracic duct). Lymph nodes act as filters removing bacteria and other particulate matter from lymph. Lymphocytes are added to lymph in these nodes.

Each group of lymph nodes has a specific area of drainage.

Each lymph node consists of a connective tissue framework; and of numerous lymphocytes and other cells, that fill the interstices of the network. The entire node is bean-shaped, the concavity constituting a hilum through which blood vessels enter and leave the node. Several lymph vessels enter the node on its convex aspect. Usually, a single lymph vessel leaves the node through its hilum (Fig. 9.4).

Microscopic Features

When a section through a lymph node is examined (at low magnification) it is seen that the node has an outer zone that contains densely packed lymphocytes, and therefore stains darkly: this part is the cortex. The cortex does not extend into the hilum.

Surrounded by the cortex, there is a lighter staining zone in which lymphocytes are fewer: this area is the medulla (Plate 9.1).

Cortex

Within the cortex there are several rounded areas that are called lymphatic follicles or lymphatic nodules. Each nodule has a paler staining germinal center surrounded by a zone of densely packed lymphocytes.

Medulla

Within the medulla, the lymphocytes are arranged in the form of branching and anastomosing cords.

Connective Tissue Framework

A lymph node is surrounded by a capsule. The capsule consists mainly of collagen fibers. Some elastic fibers and some smooth muscle may be present.

Just below the capsule is the subcapsular sinus (Fig. 9.4). A number of septa (or trabeculae) extend into the node

from the capsule and divide the node into lobules.

The hilum is occupied by a mass of dense fibrous tissue. A delicate network of reticular fibers occupies the remaining spaces forming a fibrous/reticular framework within the node. Associated with the network there are reticular cells that have traditionally been regarded as macrophages. However, it is now believed that they are

fibroblasts and do not have phagocytic properties.

Circulation of Lymph through Lymph Nodes

The entire lymph node is pervaded by a network of reticular fibers. Most of the spaces of this network are packed with lymphocytes. At some places, however, these spaces contain relatively few cells, and form channels through which lymph circulates. These channels known as sinuses are lined by endothelium, but their walls allow free movement of lymphocytes and macrophages into and out of the channels.

Afferent lymphatics reaching the convex outer surface of the node enter an extensive subcapsular sinus (Fig. 9.4). From this sinus a number of radial cortical sinuses run through the cortex toward the medulla.

Reaching the medulla the sinuses join to form larger medullary sinuses. In turn the medullary sinuses join to form (usually) one, or more than one, efferent lymph vessel through which lymph leaves the node.

Note: The afferent vessels to a lymph node enter the cortex, while the efferent vessel emerges from the medulla. The sinuses are lined by endothelium.

Lymph passing through the system of sinuses comes into intimate contact with macrophages present in the node. Bacteria and other particulate matter are removed from lymph by these cells. Lymphocytes freely enter or leave the node through these channels.

Chapter 9 Lymphatics and Lymphoid Tissue 97

Blood Supply of Lymph Nodes

Arteries enter the lymph node at the hilum. They pass through the medulla to reach the cortex where they end in arterioles and capillaries. These arterioles and capillaries are arranged as loops that drain into venules.

Postcapillary venules in lymph nodes are unusual in that they are lined by cuboidal endothelium. (They are, therefore, called high endothelial venules). This “high” endothelium readily allows the passage of lymphocytes between the bloodstream and the surrounding tissue.

These endothelial cells bear receptors that are recognized by circulating lymphocytes. Contact with these receptors facilitates passage of lymphocytes through the vessel wall.

Functions

Lymph nodes are centers of lymphocyte production. Both B-lymphocytes and T-lymphocytes are produced here by multiplication of preexisting lymphocytes. These lymphocytes pass into lymph and thus reach the bloodstream.

Bacteria and other particulate matter are removed from lymph through phagocytosis by macrophages. Antigens thus carried into these cells are “presented” to lymphocytes stimulating their proliferation. In this way lymph nodes play an important role in the immune response to antigens.

Plasma cells (representing fully mature B-lymphocytes) produce antibodies against invading antigens, while T-lymphocytes attack cells that are “foreign” to the host body.

Clinical Correlation

Infection in any part of the body can lead to enlargement and! of lymph nodes is called lymphadenitis.

Carcinoma (cancer) usually spreads from its primary site either by growth of malignant cells along lymph vessels, or by “loose” cancer cells passing through lymph to nodes into which the area drains. This leads to enlargement of the lymph nodes of the region. Examination of lymph nodes gives valuable information about the spread of cancer. In surgical excision of cancer, lymph nodes draining the region are usually removed.

THE SPLEEN

General Features

The spleen is the largest lymphoid organ of the body. Normally it is a blood-forming organ in foetal life and blooddestroying organ in postnatal life (graveyard of RBCs). Since it is in the bloodstream, it filters the blood from blood-borne antigens and microorganisms.