Lymphatic vessels intertwine with blood vessels and distribute lymphatic fluid throughout the body. They remove proteins and water from the interstitial spaces and return them to the bloodstream. Lymphatic vessels located in the small intestine are called lacteals. They absorb fats and other nutrients, producing a milky lymph fluid called chyle (from the Greek word chylos, or juice).
Chapter 5. SPLEEN
The spleen is a lymphoid organ located in the left upper quadrant of the abdomen beneath the diaphragm. The largest structure of the lymphatic system, the spleen initiates an immune response, filters and removes bacteria and other foreign substances from the bloodstream, destroys worn-out blood cells, and serves as a blood reservoir.
Chapter 6. ACCESSORY LYMPHOID ORGANS AND TISSUES
The tonsils, adenoids, appendix, thymus, and Peyer's patches remove foreign debris in much the same way lymph nodes do. They're located in food and air passages — areas where microbial access is more likely to occur.
Chapter 7. IMMUNITY
Immunity is the body's capacity to resist invading organisms and toxins, thereby preventing tissue and organ damage. The immune system's cells and organs are designed to recognize, respond to, and eliminate foreign substances, including bacteria, viruses, and parasites. The immune system also preserves the body's internal environment by scavenging dead or damaged cells and by patrolling antigens.
All foreign substances elicit the same response in general host defenses. In contrast, particular microorganisms or molecules activate specific immune responses and initially can involve specialized sets of immune cells. Such specific responses, classified as either humoral immunity or cell-mediated immunity, are produced by lymphocytes (B cells and T cells).
Chapter 8. HUMORAL IMMUNITY
In humoral immunity, an invading antigen causes B cells to divide and differentiate into plasma cells. Each plasma cell, in turn, produces and secretes large amounts of antibodies (immunoglobulin molecules that interact with a specific antigen) into the bloodstream. Antibodies destroy bacteria and viruses, thereby preventing them from entering host cells.
Five major classes of immunoglobulin exist:
· Immunoglobulin (Ig) G makes up 80% of plasma antibodies. It appears in all body fluids and is the major antibacterial and antiviral antibody.
· IgM is the first immunoglobulin produced during an immune response. It's too large to easily cross membrane barriers and is usually present only in the vascular system.
· IgA is found mainly in body secretions, such as saliva, sweat, tears, mucus, bile, and colostrum.
It defends against pathogens on body surfaces, especially those that enter the respiratory and GI tracts.
· IgD is present in plasma and is easily broken down. It's the predominant antibody on the surface of B cells and is mainly an antigen receptor.
· IgE is the antibody involved in immediate hypersensitivity reactions (or allergic reactions) that develop within minutes of exposure to an antigen. IgE stimulates the release of mast cell granules, which contain histamine and heparin.
Another part of humoral immunity, the complement system, is a major mediator of the inflammatory response. It consists of 20 proteins circulating as functionally inactive molecules. In most cases, an antigen-antibody reaction is necessary for the complement system to activate to destroy invading cells.
Chapter 9. CELL-MEDIATED IMMUNITY
In cell-mediated immunity, T-cells respond directly to antigens (foreign substances such as bacteria or toxins that induce antibody formation). This response involves destruction of target cells — such as virus-infected cells and cancer cells — through the secretion of lymphokines (lymph proteins). Organ rejection is an example of cell-mediated immunity.
Chapter 10. ACQUIRED IMMUNITY
The body readily develops long-term immunity to specific antigens, including pollen, dust, mold, and invading organisms. There are four types of acquired immunity:
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