1.  What is the function of the lymphatic system?

 The lymphatic system has more than one function.  Below are some of its major functions.

 The lymphatic system is an important part of our defense against disease.  Throughout our bodies are lymph nodes, specialized tissues located along lymphatic vessels, where white blood cells are numerous.
The white blood cells can engulf bacteria and dead or infected cells, detect invading organisms, and alert other white blood cells to the presence of some foreign protein.  Certain white blood cells can quickly make clones to mount an attack against an invader.  (See figure 14.6 in your textbook for an illustration of a lymph node.)

 No matter how well you prepare the food you eat, there are lots of bacteria on it.  As these bacteria pass through the moist warm environment of the digestive system, conditions are very favorable for them to increase greatly in number.  There are numerous lymphatic vessels and nodes around areas of the mouth and other parts of the digestive system.  They help keep in check the number of bacteria that might cause disease.  Figure 14.4a illustrates these and other areas of the body where lymph nodes are plentiful.

 In addition to controlling bacterial count, the part of the lymphatic system located in the small intestine aids in the movement of digested fats.  Microscopic finger-like structure called villi absorb digested foods.  Amino acids from protein and monosaccharides from carbohydrates are absorbed into capillaries.  However, digested fats take a little different route.  Within villi, digested fat is transported through tiny lymphatic capillaries called lacteals to larger lymphatic vessels.  Lymphatic vessels serving the lower part of the body join together to form the thoracic vessel.  This larger lymphatic vessel empties lymph into the left subclavian vein in the left shoulder region.  Figure 14.4a illustrates the location of the thoracic duct and the left subclavian vein.   Along with the blood in the vein, the fat-containing lymph circulates around the body.  When it reaches the liver, cells of the liver will process the fat to a form that can be used by body cells.

 Fluid in the body at any given time may be contained inside cells, in the plasma of the blood, or in the spaces between cells. The fluid usually shifts among these locations in response to osmotic pressure. The lymphatic system helps collect intracellular fluid into lymphatic vessels and put it back in to circulation in the blood.  Although there is no pumping “heart” that moves lymph (lymphatic fluid) through these lymphatic vessels back to where the fluid will rejoin the blood circulatory system, the massaging action of nearby muscles helps propel the lymph toward its destination.  Lymphatic valves, such as the one found in Fig. 14.3, keep the lymph moving in one direction so that it does not shift back into the area it came from.

2. What is the difference between nonspecific and specific defenses?

 As the name implies, the nonspecific defenses of the body are not aimed against a specific invader into our systems, but are more of a generalized plan of defense against invasion by some pathogen.  For example, our skin is a first line of defense in that it protects underlying organs from bacteria and, to some extent, harmful substances.  The skin acts as a barrier in several ways.  The pH of the skin is slightly acidic, which is toxic to some bacteria.  The skin’s dead top layer is rather water-proof, which prevents some substances from invading our systems.  Wherever there are openings to the outside, we have mucous membranes which also protect us against some types of invasive organisms.  Mucus produced by these membranes can trap dust & some bacteria to help further protect us.  As long as the skin and mucous membranes are intact, this gives some protection.

 In addition to these physical barriers, certain chemicals of our bodies can be included in the nonspecific defense system.  For example, hydrochloric acid is produced by the stomach.  This strong acid can help cut down on the number of bacteria that are passing though the digestive system.

 Other nonspecific defenses include the body’s ability to respond to injury with cells and chemicals that speed the healing process.  This inflammatory response attracts white blood cells to the area of an injury to phagocytize (engulf) dead cells and bacteria and to release chemicals to dilate blood vessels in the area to promote healing.

 The term “specific defenses” (immunity) refers to the body’s ability to target a particular pathogen or the toxins it is producing.  Special white blood cells are part of our specific defense system.  These include lymphocytes and macrophages.  Fig. 14.11 illustrates the pathway for development of B and T lymphocytes as they mature to the point where they can fight off specific invasive cells or chemicals.  A mature T cell will be able to directly attack cells that are recognized as “nonself” or that carry some foreign antigen.  Some of the T cells will become “memory cells”, so that they can recognize the foreign antigen at some later exposure and be able to mount an attack against it.  The B cells become an active part of fighting foreign agents by producing antibodies. Antibodies can circulate in the blood to attack antigens.  Fig. 14.13 is an illustration depicting the interaction between certain T cells and B cells as they mount an attack against an invading antigen.