Chapter 22

I. INTRODUCTION

A. The ability to ward off the pathogens that produce disease is called resistance.

B. Lack of resistance is called susceptibility.

C. Resistance to disease can be grouped into two broad areas.

1. Nonspecific resistance to disease includes defense mechanisms that provide general protection against invasion by a wide range of pathogens.

2. Immunity involves activation of specific lymphocytes that combat a particular pathogen or other foreign substance.

D. The body system that carries out immune responses is the lymphatic system.

II. LYMPHATIC AND IMMUNE SYSTEM

A. The lymphatic system consists of a fluid called lymph flowing within lymphatic vessels, several structures and organs that contain lymphatic tissue (specialized reticular tissue containing large numbers of lymphocytes), and bone marrow, which is the site of lymphocyte production.

1. Interstitial fluid and lymph are basically the same.

2. Their major difference is location.

B. The lymphatic system functions to drain interstitial fluid, return leaked plasma proteins to the blood, transport dietary fats, and protect against invasion by nonspecific defenses and specific immune responses.

C. Lymphatic Vessels and Lymph Circulation

1. Lymphatic vessels begin as blind-ended lymph capillaries in tissue spaces between cells.

a. Interstitial fluid drains into lymphatic capillaries, thus forming lymph.

b. Lymphatic capillaries merge to form larger vessels, called lymphatic vessels, which convey lymph into and out of structures called lymph nodes.

2. Lymphatic Capillaries

a. Lymphatic capillaries are found throughout the body except in avascular tissue, the CNS, portions of the spleen, and red bone marrow.

b. Lymphatic capillaries have a slightly larger diameter than blood capillaries and have overlapping endothelial cells which work as one-way valves for fluid to enter the lymphatic capillary.

c. Anchoring filaments attach endothelial cells to surrounding tissue

d. A lymphatic capillary in the villus of the small intestine is the lacteal. It functions to transport digested fats from the small intestine into blood.

3. Lymph Trunk and Ducts

a. The principal lymph trunks, formed from the exiting vessels of lymph nodes, are the lumbar, intestinal, bronchomediastinal, subclavian, and jugular trunks

b. The thoracic duct begins as dilation called the cisterna chyli and is the main collecting duct of the lymphatic system.

1) The thoracic duct receives lymph from the left side of the head, neck, and chest, the left upper extremity, and the entire body below the ribs.

2) It drains lymph into venous blood via the left subclavian vein.

c. Right Lymphatic Duct

1) The right lymphatic duct drains lymph from the upper right side of the body.

2) It drains lymph into venous blood via the right subclavian vein.

4. Formation and Flow of Lymph

a. Interstitial fluid drains into lymph capillaries.

b. The passage of lymph is from arteries and blood capillaries (blood) to interstitial spaces (interstitial fluid) to lymph capillaries (lymph) to lymphatic vessels to lymph trunks to the thoracic duct or right lymphatic duct to the subclavian veins (blood).

1) Lymph flows as a result of the milking action of skeletal muscle contractions and respiratory movements.

2) It is also aided by lymphatic vessel valves that prevent backflow of lymph.

5. An excessive accumulation of interstitial fluid may be caused by an obstruction to lymph flow (Clinical Application).

D. Lymphatic Organs and Tissues

1. Introduction

a. The primary lymphatic organs are the red bone marrow and the thymus gland that produces B and T cells.

b. The secondary lymphatic organs are the lymph nodes and spleen.

c. Included as secondary lymphatic organs are the lymphatic nodules which are clusters of lymphocytes that stand guard in all mucous membranes.

d. Most immune responses occur in secondary lymphatic organs.

2. Thymus Gland

a. The thymus gland lies between the sternum and the heart and functions in immunity as the site of T cell maturation.

b. The thymus gland is large in the infant and after puberty is replaced by adipose and areolar connective tissue.

3. Lymph Nodes

a. Lymph nodes are encapsulated oval structures located along lymphatic vessels

b. They contain T cells, macrophages, follicular dendritic cells, and B cells.

c. Lymph enters nodes through afferent lymphatic vessels, is filtered to remove damaged cells and microorganisms, and exits through efferent lymphatic vessels.

1) Foreign substances filtered by the lymph nodes are trapped by nodal reticular fibers.

2) Macrophages then destroy some foreign substances by phagocytosis and lymphocytes bring about the destruction of others by immune responses.

d. Lymph nodes are the site of proliferation of plasma cells and T cells.

e. Knowledge of the location of the lymph nodes and the direction of lymph flow is important in the diagnosis and prognosis of the spread of cancer by metastasis; many cancer cells are spread by way of the lymphatic system, producing clusters of tumor cells where they lodge. (Clinical Application)

4. Spleen

a. The spleen is the largest mass of lymphatic tissue in the body and is found in the left hypochondriac region between the fundus of the stomach and the diaphragm.

b. The spleen consists of white and red pulp.

1) The white pulp is lymphatic tissue.

a) Its T lymphocytes directly attack and destroy antigens in blood.

b) Its B lymphocytes develop into antibody producing plasma cells, and the antibodies inactivate antigens in blood.

c) Macrophages destroy antigens in blood by phagocytosis.

2) The red pulp consists of venous sinuses filled with blood and splenic cords consisting of RBCs, macrophages, lymphocytes, plasma cells, and granulocytes.

a) Macrophages remove worn-out or defective RBCs, WBCs, and platelets.

b) The spleen stores blood platelets in the red pulp.

c) The red pulp is involved in the production of blood cells during the second trimester of pregnancy.

3) The spleen is often damaged in abdominal trauma. A splenectomy may be required to prevent excessive bleeding (Clinical Application).

5. Lymphatic Nodules

a. Lymphatic nodules are oval-shaped concentrations of lymphatic tissue.

1) They are scattered throughout the lamina propria of mucous membranes lining the GI tract, respiratory airways, urinary tract, and reproductive tract.

2) This is the mucosa-associated lymphatic tissue (MALT).

b. Peyer’s patches are lymphatic nodules in the ileum of the small intestine.

c. Tonsils are multiple aggregations of large lymphatic nodules embedded in a mucous membrane at the junction of the oral cavity and the pharynx.

1) They include the pharyngeal (adenoid), palatine, and lingual tonsils.

2) They are situated strategically to protect against invasion of foreign substances and participate in immune responses by producing lymphocytes and antibodies.