AP Biology
  Notes: Lymphatic system

 A. Lymphatic System Characteristics

        1. Mammalian lymphatic system consists of lymphatic vessels and lymphoid organs.
        2. Closely associated with cardiovascular system, it has three main functions.
            a. Lymphatic vessels take up excess tissue fluid and return it to bloodstream.
            b. Lymphatic capillaries absorb fats at intestinal villi; lymphatic vessels transport fats to bloodstream.
            c. Lymphatic system helps defend the body against disease.

    B. Lymphatic Vessels

        1. Lymphatic vessels are extensive; most regions have lymphatic capillaries.
        2. Structure of larger lymphatic vessels resembles veins, including valves.
        3. Movement of fluid is dependent upon skeletal muscle contraction; when muscles contract, fluid is
            squeezed past a valve that closes, preventing it from flowing backwards.
        4. Lymphatic system is a one-way system that begins with lymphatic capillaries.
            a. They take up fluid that has diffused from but has not been reabsorbed by blood capillaries.
            b. If excess tissue fluid is not absorbed but instead accumulates, edema results.
            c. Edema is swelling caused by buildup of fluid from excessive production or inadequate drainage.
        5. Once tissue fluid enters lymphatic capillaries, it is lymph.
        6. Lymphatic capillaries join as lymphatic vessels that merge before entering one of two ducts.
            a. Thoracic duct is larger than right lymphatic duct.
                1) It serves lower extremities, abdomen, left arm, left side of head and neck, and left thoracic region.
                2) It delivers lymph to left subclavian vein of cardiovascular system.
            b. Right lymphatic duct is smaller.
                1) It serves right arm, right side of head and neck, and right thoracic region.
                2) It delivers lymph to the right subclavian vein of the cardiovascular system..

    C. Lymphoid Organs

        1. Lymphoid organs are: lymph nodes, spleen, thymus gland, bone marrow.
        2. Lymph nodes are small (about 1-25 mm) ovoid or round masses of lymphoid tissue located along lymphatic vessels.
        3. Lymph node has two regions: outer cortex and inner medulla.
        4. Lymph capillaries have macrophages that phagocytize infectious organisms and any other debris.
        5. Lymph nodes cluster in certain regions of the body (e.g., in groin and armpits).
        6. Tonsils are located in a ring around the pharynx.
            a. Adenoids are on posterior wall above border of soft palate.
            b. Larger palatine tonsils on either side of posterior oral cavity are most apt affected.
        7. Spleen is located in upper left abdominal cavity just below diaphragm.
            a. Construction of spleen is similar to that of a lymph node but it is much larger.
            b. A capsule divides spleen into lobules which contain sinuses filled blood instead of lymph.
            c. Spleen nodule contains the following:
                1) red pulp contains red blood cells, lymphocytes, and macrophages; helps to purify blood that
                    passes through by removing microorganisms and worn-out or damaged red blood cells.
                2) white pulp contains mostly lymphocytes.
            d. If spleen ruptures due to injury, it can be removed; its functions are assumed by other organs.
            e. A person without a spleen is more susceptible to infections; may require antibiotic therapy.
        8. Thymus gland is located along trachea behind sternum in upper thoracic cavity.
            a. Thymus gland is larger in children than in adults and may disappear completely in old age.
            b. It is divided into lobules by connective tissue; lobules are site of T lymphocyte maturation.
            c. Interior (medulla) of each lobule consists mostly of epithelial cells which produce thymic hormones
                (e.g., thymosin), that promote maturation of T lymphocytes.
        9. Red bone marrow originates all blood cells including all leukocytes that function in immunity.
            a. Stem cells are continually-producing cells that differentiate into various blood cells.
            b. Most bones of a child have red bone marrow; in adults it is only in skull, sternum, ribs, clavicle, pelvic
                bones and vertebral column.
            c. Red bone marrow consists of reticular fibers produced by reticular cells packed around thin-walled sinuses.
            d. Differentiated blood cells enter the bloodstream from these bone sinuses.

II. Nonspecific Defenses

    A. Immunity is ability to defend against infectious agents, foreign cells, and abnormal cancer cells.

        1. Immunity includes nonspecific and specific defenses.
        2. Four nonspecific defenses include barrier to entry, inflammatory reaction, natural killer cells, and protective proteins.

    B. Barring Entry

        1. Skin and mucous membranes lining respiratory, digestive, and urinary tracts are mechanical barriers.
        2. Oil gland secretions inhibit growth of bacteria on skin.
        3. Ciliated cells lining respiratory tract sweep mucous and particles up into throat to be swallowed.
        4. Stomach has a low pH (1.2-3.0) that inhibits growth of many bacteria.
        5. Normal bacteria that reside in intestine or vagina prevent pathogens from colonizing.
        6. Pathogens are disease causing agents (viruses or bacteria).

    C. Inflammatory Reaction

        1. If skin is broken, a series of events occurs: the inflammatory reaction.
        2. The inflamed area has four symptoms: redness, pain, swelling, and heat.
        3. Mast cells occur in tissues and resemble basophils.
        4. When tissue damage occurs, a capillary and several tissue cells rupture and release bradykinin.
            a. Bradykinin triggers nerve impulses in pain receptors and stimulates mast cells to release histamine.
            b. Bradykinin and histamine cause vasodilation and increased permeability of capillaries.
            c. Enlarged capillaries produce redness and local increase in temperature.
            d. A rise in local temperature reduces invading pathogens and increases phagocytosis by WBCs.
        5. Chemicals released by damaged tissue cause neutrophils and monocytes to migrate by amoeboid movement
            to site of injury; they escape from blood by squeezing through capillary wall.
        6. When monocytes enter tissue, they differentiate into macrophages that ingest bacteria or viruses.
        7. Connective and lymphoid tissues have resident macrophages that devour old blood cells and debris.
        8. Macrophages trigger an explosive increase in leukocytes by releasing colony-stimulating hormones; this
            diffuses into blood and is transported to red bone marrow to stimulate production of WBCs.
        9. Pus is accumulation of dead neutrophils along with tissue, cells, bacteria and, living WBCs.
        10. Aspirin, ibuprofen, and cortisone are anti-inflammatory agents that counter inflammatory chemistry.

    D. Natural Killer Cells

        1. Natural killer cells kill virus-infected cells and tumor cells; they lack specificity and memory.
        2. Complement system, called complement, is plasma proteins designated by letter C and a subscript.
            a. One activated complement protein activates another protein in set series of domino reactions.
            b. A limited amount of protein can activate many other proteins.
            c. Complement is activated when pathogens enter the body.
            d. It "complements" certain immune responses, which accounts for its name.
            e. It amplifies an inflammatory reaction by attracting phagocytic cells to site of infection.
            f. Complement binds to antibodies already on the surface of pathogens, increasing probability that pathogens
                will be phagocytized by a neutrophil or macrophage.
            g. Some complement proteins form membrane attack complex that produces holes in bacterial cell walls
                and plasma membranes; fluids and salts enter to point where they burst.
        3. Interferon is protein produced by virus-infected animal cells.
            a. It binds to receptors of non-infected cells, producing substances interfering with viral replication.
            b. Interferon is specific to a species; only human interferon can be used in humans.

III. Specific Defenses

    A. If nonspecific defenses fail, specific defenses are required against a particular antigen.

        1. Antigens are foreign substances, protein or polysaccharide, that stimulate immune system to react.
        2. Pathogens have antigens; antigens can also be components of foreign or cancer cells.
        3. We do not ordinarily become immune to our own cells; immune system can tell self from nonself.
        4. Immunity usually lasts for some time; we do not ordinarily get the same illness a second time.

    B. Specific immunity is primarily result of action of B lymphocytes and T lymphocytes.

        1. B lymphocytes mature in bone marrow; T lymphocytes mature in thymus.
        2. B lymphocytes (B cells) give rise to plasma cells that produce antibodies.
        3. Antibodies are large globular proteins that combine with and neutralize antigens.
        4. Antibodies are secreted into blood and lymph.
        5. T lymphocytes either directly attack cells that bear antigens or regulate immune response.
        6. Lymphocytes are capable of recognizing an antigen; they have receptor molecules on their surface.
            a. Receptor-antigen fit is compared to a lock and key.
            b. During our lifetime, we encounter a million different antigens; we need diversity of lymphocytes.
            c. During maturation, diversification produces a different lymphocyte for each possible antigen.