Ch. 7: The Lymphatic System and Immunity
(text pp. 145-162)
Human Biology (BIOL100)

The Lymphatic System is composed of a network of lymphatic vessels tranporting a fluid derived from blood called lymph fluid. In addition to the vessels the Lymphatic System also includes lymphoid organs in which specialized leukocytes called lymphocytes are produced. Lymphocytes form an important part of immunity, which is the ability of the body to defend against pathogens and/or their toxins, foreign cells, and abnormal body cells such as cancer cells. A pathogen is any disease-causing agent (eg. some types of viruses and bacteria):
1) atoms.
2) molecules.
3) cell organelles.
4) cells.
5) tissues.
6) organs.
7) organ systems:
a. The Digestive System.
b. The Cardiovascular System.
c. The Lymphatic System and Immunity.


I. The Lymphatic System
The Lymphatic System consists of lymphatic vessels and lymphoid organs.
The primary functions of the Lymphatic System are:
1) Transporting excess interstitial fluid from the tissues back to the Cardiovascular System.
2) Absorbing fats in the intestinal villi and transporting them to the Cardiovascular System.
3) Producing a type of leukocyte called lymphocytes which provide immunity. Two sub-classes of lymphocyte are produced and spread throughout the body:
a. T lymphocytes.
b. B lymphocytes.

A. Lymphatic Vessels (fig. 7.2)
Transportation of lymph fluid as described above.

B. Lymphoid Organs (fig. 7.3)
Produce lymphocytes.
The lymphoid organs are:
1) Lymph nodes - lymph fluid filters through lymph nodes and pathogens and/or their toxins are removed by lymphocytes. The tonsils are lymph nodes.
2) Spleen - blood filters through the spleen and pathogen and/or their toxins are removed by lymphocytes.
3) Red Bone Marrow - produces B lymphocytes.
4) Thymus - produces T lymphocytes.


II. Immunity
Immunity is the ability of the body to defend itself against pathogens and/or their toxins, foreign cells, and abnormal cells.
Immunity includes both non-specific and specific defenses.

A. Non-specific Defenses (figs. 7.4 and 7.5)
The first line of defense.
Immediately respond to any type of pathogen that enters the body.
There are 4 types of non-specific defense mechanisms:
1) Barring Entry - Skin, mucous membranes, oil glands, lysozyme in tears, and stomach acid all contribute to preventing pathogens from entering the body.
2) Inflammatory Reaction - Redness, heat, swelling, and pain characterize the inflammatory reaction. The release of histamine and kinins from damaged tissue cells and leukocytes called mast cells bring about redness and swelling. A rise in temperature increases phagocytosis by leukocytes called neutrophils and macrophages. The swollen area stimulates free nerve endings, causing the sensation of pain.
3) Natural Killer (NK) Cells - These are lymphocytes that destroy virus-infected cells and cancer cells by cell-to-cell contact.
4) Protective Proteins - Plasma proteins called complement attract phagocytes, enhance phagocytosis, and disrupt bacterial cell membranes. Interferons are secreted by virus-infected cells.

B. Specific Defenses
The second line of defense.
Delayed immune response directed against a specific type of antigen.
An antigen is any molecule (usually protein) that can stimulate an immune response. Pathogens, foreign cells, and cancer cells carry antigens.
There are 2 branches of the specific defenses:
1) B lymphocytes and Antibody-Mediated Immunity.
2) T lymphocytes and Cell-Mediated Immunity.

1. B lymphocytes and Antibody-Mediated Immunity (figs. 7.6 and 7.7)
B lymphocytes are produced in the red bone marrow.
Antibodies are protein produced by B lymphocytes that bind antigen.

The Clonal Selection Theory states that B lymphocytes randomly form millions of different antibody receptors during fetal development.
If a particular antigen enters the body the B lymphocytes carrying the matching antibody receptor will bind the antigen and undergo clonal expansion.
Clonal expansion results in 2 types of B lymphocyte being produced:
a. Plasma cells - Secrete antibodies which bind to the antigen. This results in the pathogen carrying the antigen being marked for phagocytosis or complement destruction.
b. Memory cells - Provide long-term immunity.

2. T lymphocytes and Cell-Mediated Immunity (figs. 7.8 and 7.9)
T lymphocytes are produced in the thymus.
The two primary types of T lymphocyte are:
a. Cytotoxic T cells - Destroy cells that carry antigen by cell-to-cell contact with perforin.
b. Helper T cells - Stimulate both Antibody- and Cell-Mediated Immunity. These cells are infected and destroyed by the HIV virus resulting in the loss of normal specific defenses.


III. Induced Immunity
A. Active Immunity (fig. 7.10)
Vaccines induce long-term active immunity against pathogens by exposing the body to antigens carried by pathogens.
This creates a primary immune response with the production of memory cells.
Memory cells provide long-term immunity.

B. Passive Immunity
Antibodies can be transferred to an individual to provide passive immunity.
Mothers milk contains antibodies.


IV. Immunity Side Effects
A. Allergies
B. Autoimmune Diseases
C. Tissue Rejection