The Lymphatic System and Immunity

Functions of the Lymphatic System The lymphatic system has 3 main functions: Drains excess interstitial fluid and returns it to the blood Transports lipids and lipid-soluble vitamins (A, D, E, and K) absorbed by the GI tract to the blood Carries out specific immune responses against certain foreign invaders (microbes, etc.)

Lymphatic System The lymphatic system consists of a fluid called lymph, lymphatic vessels, lymphatic organs (such as red bone marrow, the thymus, lymph nodes, and the spleen), and other lymphatic tissues This system helps in circulating body fluids and defending the body against disease-causing organisms or substances

Lymph Most components of blood plasma filter out through capillary walls to form interstitial fluid This fluid then passes into lymphatic capillaries and is called lymph; lymph is usually a clear, pale yellow fluid Interstitial fluid is located in between cells; lymph is located within lymphatic vessels and in lymphatic tissue

Lymphatic Vessels Lymphatic capillaries—tiny vessels located in the spaces between cells; closed at one end Lymphatic capillaries unite to form larger lymphatic vessels, which are similar to veins, except they have thinner walls and more valves

Lymphatic Vessels (continued) As lymphatic vessels exit lymph nodes, they unite to form lymphatic trunks and then lymphatic ducts There are 2 main lymphatic ducts: Thoracic duct (or left lymphatic duct)—the main duct for the return of lymph to the blood; receives lymph from the left side of the body and the entire lower body and drains it into venous blood near the left internal jugular and left subclavian veins Right lymphatic duct—receives lymph from the upper right side of the body and drains it into venous blood near the right internal jugular and right subclavian veins

Red Bone Marrow Stem cells in red bone marrow give rise to B cells and pre-T cells Pre-T cells migrate to the thymus, where they develop into mature T cells Some of these T cells leave the thymus and travel through the bloodstream to lymph nodes, the spleen, and other lymphatic tissues

Thymus The thymus is a lymphatic organ located in the mediastinum between the aorta and the sternum It is large in infants, but it atrophies after puberty is reached Most T cells are formed before puberty, but some continue to develop in the thymus throughout an individual’s lifetime

Lymph Nodes Lymph nodes are bean-shaped organs consisting of masses of B cells and T cells that have migrated through the blood They are scattered throughout the body and are located at intervals along lymphatic vessels Lymph passes through lymph nodes and is filtered as it travels through the lymphatic vessels

Spleen The spleen is a lymphatic organ located in the left side of the abdominal cavity near the stomach and diaphragm It contains B cells and T cells and carries out immune functions related to these cells In addition, the macrophages in the spleen remove pathogens and worn out/dead/damaged blood cells from the blood The spleen stores up to 1/3 of the body’s supply of platelets During fetal life, it produces blood cells

Other Lymphatic Tissues The appendix contains some lymphatic tissue The tonsils are also composed of lymphatic tissue and are located in the pharynx Usually there are 5 tonsils: Adenoid (or pharyngeal tonsil)—a single tonsil in the nasopharynx Palatine tonsils—two tonsils located at the rear of the oral cavity; the ones usually removed during a tonsillectomy Lingual tonsils—two tonsils located at the base of the tongue

Resistance Resistance is the natural ability of the body to fight damage or disease There are 2 types of resistance: Nonspecific resistance Specific resistance or immunity

Nonspecific Resistance Nonspecific resistance consists of defense mechanisms that are present at birth and give immediate protection against pathogens and foreign substances It includes the barriers provided by the skin and mucous membranes, lacrimal fluid (or tears), stomach acid, saliva, perspiration, phagocytes (neutrophils and macrophages), natural killer cells, inflammation (causes redness, pain, heat, and swelling), and fever

Specific Resistance: Immunity The ability of the body to defend itself against specific invading organisms or substances is called specific resistance or immunity Substances that are recognized as foreign and cause immune responses are called antigens In autoimmune diseases, the immune system attacks the body’s own tissues (examples—multiple sclerosis, type I diabetes, rheumatoid arthritis, and lupus)

Major Histocompatibility Complex Most body cells contain “self antigens” in their plasma membranes These antigens make up the major histocompatibility complex (MHC) and are also called human leukocyte antigens (HLA) This complex of antigens consists of thousands of molecules that vary from one individual to another

MHC (continued) Only identical twins have the same antigens of this type When “matching” is done for bone marrow or other organ transplants, a donor and recipient are compared with respect to their MHC antigen compositions

T Cells Depending on the antigen receptors they have, T cells are classified as either CD4 or CD8 cells They must recognize an antigen and be “activated” before they can eliminate foreign invaders

T Cells (continued) Once activated, T cells divide, differentiate, and produce 3 specific types of cells: Helper T cells—most CD4 cells develop into these cells, which cause both B cells and T cells to proliferate Cytotoxic T cells—CD8 cells develop into these cells, which leave lymphatic tissues to destroy infected cells (or cancerous cells) by digesting them or causing them to rupture and die Memory T cells—these cells “remember” antigens that have previously invaded the body and can quickly eliminate them

B Cells In addition to the MHC antigens, the plasma membranes of B cells and T cells have antigen receptors that enable them to recognize foreign antigens and respond to them After recognizing an antigen and being “activated”, B cells transform into plasma cells and begin secreting antibodies (or immunoglobulins) Memory B cells can recognize antigens that have previously invaded the body and quickly eliminate them

Antibodies Antibodies are chemically glycoproteins (carbohydrates attached to proteins) also known as immunoglobulins The structure of an antibody matches its antigen much as a lock accepts a specific key Antibodies travel through lymph and can inactivate specific foreign antigens by binding with them Some antigen-antibody complexes are destroyed by phagocytic cells

Antibodies There are 5 classes of antibodies: IgG—most abundant; about 80% of all antibodies in the blood; can cross the placenta from mother to fetus, giving newborns immune protection IgA—makes up 10-15% of antibodies in the blood; levels decrease during stress IgM—makes up 5-10% of antibodies in the blood; first secreted by plasma cells IgD—makes up only about 0.2% of antibodies in the blood IgE—makes up less than 0.1% of all antibodies in the blood; involved in allergic reactions