7.2 Lymph Nodes

Scattered along the Iymphatic vessels, like beads on a string, are small (l to 25 mm in length), bean-shaped masses of tissue called Iymph nodes. Look at the lymph node diagram in image 7.2a and the light microscope photo in image 7.2b. The nodes are found in the largest concentrations at the neck, armpit, thorax, abdomen, and groin. Lesser concentrations are found behind the elbow and knee. The superficial Iymph nodes are located near the body surface in the neck, armpit, and groin. The deep nodes are located deep within the groin area, near the lumbar vertebrae, at the base of the lungs, attached to the tissue surrounding the small intestines, and in the liver. Most Iymph passes through at least one Iymph node on its way back to the bloodstream.

Afferent (to the node) Iymphatic vessels with Iymph enter the node at various points along the outer capsule that surrounds the node. Efferent (from the node) Iymphatic vessels leave the node from a small depressed area called the hilus. Blood vessels enter, as well as leave, through the hilus.

Lymph nodes filter out harmful microorganisms and other foreign substances from the Iymph, trapping them in a mesh of reticular fibers. Lymph nodes are also the initiating sites for the specific defenses of the immune response. Lymph nodes are covered by a capsule of fibrous connective tissue (black in image 7.2a) and projections of this connective tissue called trabeculae extend inward from the capsule toward the center of the Iymph node, dividing it into compartments. The outer portion of each compartment is the cortex of the node (labeled "a" in image 7.2b) located directly underneath the subscapular sinus (arrow in image 7.2b). The inner part of a Iymph node is the medulla (labeled "b" in image 7.2b).

7.3 Cortex of Lymph Nodes

The outer portion of a lymph node is called the cortex, as labeled as "a" in image 7.3a. It contains Iymphocytes in dense clusters called Iymph nodules. In the middle of each nodule is the germinal center, where Iymphocytes are produced by cell division. Image 7.3b shows a germinal center (labeled "a" in image 7.3b). The Iymph nodes produce about 10 billion Iymphocytes every day.

There are two types of lymphocytes--B and T lymphocytes. In lymph nodes, B lymphocytes are found primarily in the germinal centers, where they differentiate into plasma cells. Plasma cells produce antibodies that act against particular foreign substances. Image 7.3c shows a plasma cell at the tip of the yellow arrow. The remaining cortical cells are T lymphocytes, which attack specific foreign cells.

The inner part of a Iymph node is the medulla (labeled "Me" in image 7.3d); the outer part is the cortex (labeled "Co" in image 7.3d); and the entire organ is covered with a fibrous capsule (labeled "Ca" in image 7.3d). Immediately below the capsule is the subscapular sinus (labeled "SS" in image 7.3d). Also seen are the lymphatic nodules (labeled "LN" in image 7.3d). Strands of Iymphocytes extend from these nodule and these strands are appropriately called the medullary cords (labeled "MC" in image 7.3d). Running from the cortex to the medulla, and surrounding the nodules and medullary cords, are medullary sinuses (labeled "MS" in image 7.3d), through which the Iymph flows before it leaves the node via efferent lymphatic vessels (labeled "LV" in image 7.3d). The Iymph node effectively funnels foreign materials in the Iymph through the sinuses so that the Iymph comes in contact with Iymphocytes and macrophages. At the bottom of the image in the area of the hilus (labeled "Hi" in image 7.3d), and artery (labeled "Ar" in image 7.3d) and a vein (labeled "Ve" in image 7.3d) is shown.

7.4 Spleen

In addition to Iymphatic capillaries, Iymphatics, and Iymph nodes, the Iymphatic system consists of Iymphoid organs: the spleen and thymus gland. The spleen is the largest Iymphoid organ in the body. It is about the size and shape of a small clenched fist, measuring about 12 cm in length, and is purplish in color. Located below the diaphragm on the left side of the body, it rests on portions of the stomach, kidney, and large intestine. The main functions of the spleen are filtering blood and manufacturing phagocytic Iymphocytes and monocytes. The spleen also serves as a blood reservoir. If the body loses blood suddenly, the spleen contracts and adds blood to the general circulation. It also relieves the venous pressure on the heart by releasing stored blood into the circulation during bursts of physical activity. The spleen is capable of releasing approximately 200 mL of blood into the general circulation in one minute.

The general anatomy of the spleen (image 7.4a) is similar to that of Iymph nodes. Surrounded by a capsule of connective tissue, the spleen is divided by trabeculae into compartments called lobules. The functional part of the medulla consists of splenic pulp, which contains small islands of white pulp scattered throughout red pulp. The light microscope photo in image 7.4b, shows red pulp (labeled "a" in image 7.4b) and white pulp (labeled "b" in image 7.4b).

Image 7.4c is a close-up view of the splenic pulp. The white pulp is made up of compact masses of Iymphocytes surrounding small branches of the splenic artery (labeled "b" in image 7.4c). These masses, which occur in intervals, are called splenic nodules (Malpighian corpuscles). Inside the white pulp are germinal centers, where lymphocytes are produced. The area labeled "c" in image 7.4c is the red pulp which surrounds the splenic nodules and is itself penetrated by numerous sinusoids (labeled "a" in image 7.4c). `

Image 7.4d is a germinal center. Note the numerous white blood cells (black dots in image 7.4d) arranged in a circular mass. A small branch of the splenic artery can be seen as a small red cylinder towards the right of the germinal center. The majority of the center is a loose mass of actively dividing cells.

Image 7.4e shows a venous sinusoid (labeled "a" in image 7.4e) which is filled with blood and lined with monocytes and macrophages (labeled "b" at tip of arrow in image 7.4e). The pulp is red because of the many erythrocytes in the blood. This arrangement brings the blood and any foreign materials it may contain to the Iymphocytes, monocytes, and macrophages for cleansing. Since the spleen does not receive Iymphatics and Iymph, it cannot be considered a filter in the same way that a Iymph node is a filter; instead, it detoxifies the blood directly.