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Subject: General Science

Topic: Lymphatic System and Immunity

THE LYMPHATIC SYSTEM

• The lymphatic system is composed of lymph vessels, lymph nodes, and organs. The functions of this system include the absorption of excess fluid and its return to the blood stream, absorption of fat (in the villi of the small intestine) and the immune system function.

• Lymph vessels are closely associated with the circulatory system vessels. Larger lymph vessels are similar to veins. Lymph capillaries are scattered throughout the body. Contraction of skeletal muscle causes movement of the lymph fluid through valves.

• Lymph organs include the bone marrow, lymph nodes, spleen, and thymus.

• Bone marrow contains tissue that produces lymphocytes. B-lymphocytes (B-cells) mature in the bone marrow.

• T-lymphocytes (T-cells) mature in the thymus gland.

• Other blood cells such as monocytes and leukocytes are produced in the bone marrow.

• Lymph nodes are areas of concentrated lymphocytes and macrophages along the lymphatic veins.

• The spleen is similar to the lymph node except that it is larger and filled with blood.

• The spleen serves as a reservoir for blood, and filters or purifies the blood and lymph fluid that flows through it.

• If the spleen is damaged or removed, the individual is more susceptible to infections.

• The thymus secretes a hormone, thymosin, that causes pre-T-cells to mature (in the thymus) into T-cells.

IMMUNITY

• Immunity is the body’s capability to repel foreign substances and cells.

• The nonspecific responses are the first line of defense.

• Highly specific responses are the second line of defense and are tailored to an individual threat.

• The immune response includes both specific and nonspecific components. Nonspecific responses block the entry and spread of disease-causing agents.

• Antibody-mediated and cell-mediated responses are two types of specific response.

• The immune system is associated with defense against disease-causing agents, problems in transplants and blood transfusions, and diseases resulting from over-reaction (autoimmune, allergies) and under- reaction (AIDS).

(A) GENERAL DEFENCES

Barriers to entry are the skin and mucous membranes.

1. The skin is a passive barrier to infectious agents such as bacteria and viruses. The organisms living on the skin surface are unable to penetrate the layers of dead skin at the surface. Tears and saliva secrete enzymes that break down bacterial cell walls. Skin glands secrete chemicals that retard the growth of bacteria.

2. Mucus membranes lining the respiratory, digestive, urinary, and reproductive tracts secrete mucus that forms another barrier. Physical barriers are the first line of defense.

3. When microorganisms penetrate skin or epithelium lining respiratory, digestive, or urinary tracts, inflammation results. Damaged cells release chemical signals such as histamine that increase capillary blood flow into the affected area (causing the areas to become heated and reddened). The heat makes the environment unfavorable for microbes, promotes healing, raises mobility of white blood cells, and increases the metabolic rate of nearby cells. Capillaries pass fluid into injured  areas, causing the infected/ injured area to swell.

4. Clotting factors trigger formation of many small blood clots. Finally, monocytes (a type of white blood cell) clean up dead microbes, cells, and debris.

5. If this is not enough to stop the invaders, the complement system and immune response act.

6. Protective proteins that are produced in the liver include the complement system of proteins. The complement system proteins bind to a bacterium and open pores in its membrane through which fluids and salt move, swelling and bursting the cell. The complement system directly kills microbes, supplements inflammatory response, and works with the immune response. It complements the actions of the immune system. Complement proteins are made in the liver and become active in a sequence (C1 activates C2, etc.). The final five proteins form a membrane attack complex (MAC) that embeds itself into the plasma membrane of the attacker.

7. Salts enter the invader, facilitating water to cross the membrane, swelling and bursting the microbe. Complement also functions in the immune response by tagging the outer surface of invaders for attack by phagocytes.

8. Interferon is a species-specific chemical produced by cells that are viral attack. It alerts nearby cells to prepare for a virus. The cells that have been contacted by interferon resist all viral attacks.