Lymphatic Capillaries, Lymphatic Vessels, and Lymphatic Trunks and Collecting Ducts
Tissue Fluid Formation and Lymph Formation and Function
Structure of a Lymph Node, Locations of Lymph Nodes, and Functions of Lymph Nodes
Body Defenses Against Infection
Species Resistance, Mechanical Barriers, Chemical Barriers, Fever, Inflammation, and Phagocytosis
Antigens, Lymphocyte Origins, Lymphocyte Functions, T Cell Activation, and B Cell Activation
Types of Antibodies and Antibody Actions
Types of Acquired Immunity and Allergic Reactions
The lymphatic system is made of a network of connecting vessels that collect fluids between cell. These lymphatic vessels then return this fluid (called lymph) to the blood stream. The lymphatic system also picks up lipids from the digestive organs and transports them to the blood stream. Finally the lymphatic system functions to defend our bodies against pathogens (disease-causing agents).
The lymphatic pathways start with tiny vessels called lymphatic capillaries. The lymphatic capillaries merge together to make lymphatic vessels. Eventually lymphatic vessels merge together to make lymphatic trunks and the trunks merge into lymphatic collecting ducts.
Lymphatic capillaries extend into the spaces between cells called interstitial spaces. They have very permeable, thin walls that are designed to pick up fluids in interstitial spaces. Once fluid enters lymphatic capillaries, it is called lymph.
Lymphatic capillaries deliver lymph to lymphatic vessels. Lymphatic vessels deliver the lymph to lymph nodes. The cells inside lymph nodes can remove pathogens from lymph or start an immune response against the pathogen. The immune response is discussed later in this chapter. Lymph leaves lymph nodes through lymphatic vessels.
Lymphatic vessels eventually deliver lymph to larger lymphatic trunks. The trunks deliver the lymph to even larger lymphatic collecting ducts. There are two major lymphatic collecting ducts in the body - 1) the thoracic duct and 2) the right lymphatic duct. Both of these ducts empty lymph into large veins of the body in the thoracic cavity.
The right lymphatic duct is much smaller than the thoracic duct. The right lymphatic duct collects all the lymph from the right side of the head and neck, right arm and right side of the thoracic part of the body. The thoracic duct collects lymph from the left side of the head and neck, left arm, left side of the thorax, entire abdominopelvic area, and both legs!
Click here for a summary of the lymphatic pathway
Click here for a summary of the lymphatic system
Click here to view areas drained by different lymphatic ducts
Remember that lymph is basically just tissue fluid that has entered a lymphatic capillary.
Fluid leaks out of blood capillaries into spaces between cells. This fluid is high in nutrients, oxygen, and small proteins. Most of this fluid is picked up by body cells. However, some of the fluid persists between cells. This is called tissue fluid that is destined to become lymph.
As the small proteins stay in the spaces between cells, they increase the osmotic pressure in these spaces. So, water will be drawn to the proteins. Then the hydrostatic pressure increases between the cells and this pressure forces the tissue fluid into lymphatic capillaries. So eventually the lymph will end up returning these small proteins and fluids back to the blood stream.
Once lymph enters lymphatic vessels it is pushed through the vessels by the squeezing action of neighboring skeletal muscles. Lymphatic vessels also contain valves that prevent the backflow of lymph once it has been squeezed through the vessels. Breathing movements also squeeze lymphatic vessels and therefore promote lymph movement.
If lymph is not moved through a lymphatic vessel then it will leak back out of the lymphatic capillaries. When this happens swelling of the surrounding tissue occurs. This condition is also called edema.
Lymph nodes are very small glandular structures that usually cannot be felt very easily. One side of a lymph node is indented; this indention is called a hilum. Nerves and blood vessels enter the node through the hilum.
Lymphatic vessels carry lymph to a lymph node on surface away from the hilum. Lymphatic vessels that carry lymph to the node are called afferent (afferent meaning "to") vessels. About four or five afferent vessels are associated with each node. Lymphatic vessels that carry lymph out of a node are called efferent (efferent meaning "away from") vessels. A lymph node usually only has one or two efferent vessels. Because more lymph enters the node than can exit at one time, lymph tends to pool or stay in the node for some period of time.
Two important cell types are found inside the node - 1) macrophages and 2 ) lymphocytes. Macrophages will digest unwanted pathogens in the lymph as it sits in the node and the lymphocytes will start an immune response against the pathogen.
Lymph nodes are located along the paths of larger lymphatic vessels. They are spread throughout the body but do not occur in the nervous system.
Click here for the structure and distribution of lymph nodes.
As mentioned above, lymph nodes digest unwanted pathogens in lymph through the action of their macrophages and mount immune responses against pathogens through the action of their lymphocytes. Lymph nodes are also responsible for the generation of some lymphocytes. Recall that the bone marrow also makes lymphocytes.
The thymus is a soft, bilobed organ located just above the heart. As a person ages, the thymus shrinks. The thymus carries out the same functions as a lymph node but is also responsible for the production of T lymphocytes. Recall that the thymus also makes the hormone called thymosin.
The spleen is the largest lymphatic organ. It is located in upper left portion of the abdominal cavity. The spleen is filled with blood, macrophages and lymphocytes and therefore filters blood much the same way lymph nodes filter lymph. The spleen also removes worn out red blood cells from blood.
Click here for the locations of the thymus and spleen
An infection is the presence of a pathogen in or on the body. A pathogen is a disease-causing agent such as a bacterium, virus, toxin, fungi, or protozoan. The body has mechanisms to protect us against pathogens in general; these mechanisms are called nonspecific defenses. The body also has mechanisms to protect us against very specific pathogens; these mechanisms are called immunities.
The following are nonspecific mechanisms to protect our bodies against pathogens:
Species resistance simply means that a species typically only gets diseases that are unique to the species. For example, humans do not get diseases that affect plants. Humans also do not get most diseases that affect other animals.
The covering of the body (skin) and linings of the tubes of the body (mucous membranes) provide mechanical barriers against pathogens.
Chemicals and enzymes in body fluids provide chemical barriers that destroy pathogens. For example, acid in the stomach juices destroy pathogens that we swallow. Lysozymes in tears destroy pathogens on the surface of the eye. Salt in sweat also kills bacteria. Interferon in blood blocks viruses from infecting cells.
An elevated body temperature is a fever. Fever causes the liver and spleen to take iron out of the blood stream. Many pathogens need iron to survive in out bodies so when their iron sources are gone, they die. Fever also activates phagocytic cells in the body to attack pathogens.
When an area of the body becomes injured or infected with a pathogen, inflammation can result. In inflammation, blood vessels in the injured area dilate and become leaky. Therefore, more blood enters the area. The blood brings phagocytic white blood cells to the area to attack pathogens. The blood also brings proteins to replace injured tissue. The blood also brings clotting factors to the area. The clotting factors "wall off" the area so pathogens cannot spread.
Click here for a summary of events of inflammation
Recall neutrophils and monocytes are the most active phagocytes in blood. They can also leave the blood stream to attack pathogens in other tissue. When a monocyte leaves the blood stream, it can become a macrophage which is a larger phagocytic cell.
Click here for a summary of nonspecific body defenses
Specific defenses are also called immunities. Different types of lymphocytes and macrophages are the major cells involved in specific defenses.
An antigen can very simply be thought of as a foreign substance in the body. Antigens are found on the surface of pathogens. The body is programmed to recognize antigens in the body.
Antigens usually start an immune response in the body. However, if the antigens are not large enough, they may not start an immune response. Foreign substances in the body too small to start an immune response by themselves are called haptens.
There are two types of lymphocytes - 1) B cells and 2) T cells. Most of the lymphocytes in blood are T cells. Both B cells and T cells circulate in the blood; they are also found in lymph nodes, the spleen, the thymus, the lining of digestive organs, and bone marrow.
T cells and B cell both recognize antigens in the body. However they respond to antigens in different ways.
T cells tend to bind to antigens on cells and attack them directly. This type of response is called cell-mediated.
B cells do not attack antigens directly. B cells respond to antigens by becoming plasma cells. The plasma cells then make antibodies against the specific antigen the B cell responded to. The antibodies end up attaching to antigens in the humors (fluids) of the body so this response is called a humoral response.
T cells respond to antigens also by secreting different type of chemicals called cytokines. Some cytokines increase T cell production, some cytokines increase B cell production, some cytokines directly kill cells with antigens and some cytokines stimulate the bone marrow to produce more white blood cells.
Before a T cell can respond to an antigen, it must be activated. T cell activation begins when a macrophage ingests and digests a pathogen that has antigens on it. The macrophage then takes some of the antigens from the pathogen and puts them on its cell membrane next to a large protein complex called a major histocompatibility complex (MHC). A T cell that has a receptor for the antigen recognizes and binds to the antigen and MHC on the surface of the macrophage; the t cell is now activated and becomes a helper T cell which can respond to the antigen.
Some T cells are called cytotoxic T cells. They are activated in similar ways as describe above except they primarily respond to antigens and MHCs on the surface of cancerous cells and viruses. So, they are responsible for getting rid of cancer cells and viruses in the body.
Still some T cells are activated to become memory T cells. The memory cells are activated the next time a person is exposed to the same antigen.
A B cell becomes activated when a specific antigen binds to a receptor on its surface. A B cell will only recognize one type of antigen. The B cell will then divide to make plasma cells and memory cells. The plasma cells start to make antibodies; the antibodies then go out into the fluids of the body and bind to the same types of antigens that activated the B cell. The memory cells are activated the next time the person is exposed to the same antigen.
Click here to view B cell activation
Click here for a comparison table of B cells and T cells
Antibodies are also called immunoglobulins. The following is a list of different type of immunoglobulins (Ig):
IgG - this antibody primarily recognizes different types of bacteria, viruses, and toxins; it can also activate complement (this is described later)
IgA - this is the antibody type found in secretions of the body such as breast milk, sweat, tears, saliva and mucus.
IgM - IgM is a very large antibody that primarily binds to antigens on food, bacteria or incompatible blood cells. It can also activate complement.
IgE - this type is found wherever IgA is found. It is involved in allergic reactions.
When an antibody binds to an antigen, it does one of the following things:
1) It causes a direct attack of the antigen. When some antibodies bind to antigens, it causes the antigens to clump together. When antigens are clumped together, it is easier for phagocytes to attack and destroy them.
2) Antibodies can cover the toxic portions of antigens to render them untoxic.
3) Antibodies can activate complement. Complement is a group of proteins that attack pathogens by forming holes in them. Complement proteins also attract macrophages to pathogens and can stimulate inflammation.
Click here for a summary table of antibody actions
A primary immune response occurs the first time a person is exposed to an antigen. This response is slow and takes several weeks to occur. In this response, memory cells are made.
A secondary immune response occurs the second, third, fourth, etc, time a person is exposed to the same antigen. This response is very quick and usually prevents a person from developing a disease from the antigen. Memory cells carry out the secondary immune response.
There are four types of immunities we acquire as follows:
1) Naturally acquired active immunity - we develop this immunity when we are naturally exposed to an antigen and subsequently make antibodies against the pathogen; when we get a disease we develop this type of immunity; this type is long-lived usually.
2) Artificially acquired active immunity - we develop this immunity when we are injected with a pathogen and then subsequently make antibodies against the pathogen; a vaccine induces this type of immunity; this type is long-lived usually.
3) Naturally acquired passive immunity - we are given this immunity through our mothers; when our mother passes antibodies to us through the placenta or through breast milk this is called naturally acquired passive immunity; this type of immunity is short-lived.
4) Artificially acquired passive immunity - we are given this immunity when someone injects us with antibodies; if you bitten by a snake, a physician will inject you with antibodies (antivenom) to neutralize the venom which is an antigen; this is a short-lived immunity.
Click here for a summary table of acquired immunities
An allergic reaction is an immune response to a nonharmful substance such as pollen or an excessive immune response. Substances that trigger allergic responses are called allergens. Allergic reactions are caused by IgE antibodies. These antibodies bind to allergens and then cause mast cells to release substances that trigger allergic responses.