1. The Lymphatic System and Immunity

2. 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.)

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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)

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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