OVERVIEW OF THE LYMPHATIC SYSTEM / 440 CELLS OF THE LYMPHATIC SYSTEM / 441 Overview / 441 Lymphocytes / 444 Antigen-Presenting Cells / 453 LYMPHATIC TISSUES AND ORGANS / 453 Lymphatic Vessels / 453 Diffuse Lymphatic Tissue and Lymphatic Nodules / 456 Lymph Nodes / 460 Cells of the Reticular Meshwork / 461 Thymus / 466 Spleen / 471 Folder 14.1 Functional Considerations: Origin of the Names T Lymphocyte and B Lymphocyte / 447 Folder 14.2 Clinical Correlation: Hypersensitivity Reactions / 447 Folder 14.3 Clinical Correlation: Human Immunodeficiency Virus (HIV) and Acquired Immunodeficiency Syndrome (AIDS) / 455 Folder 14.4 Clinical Correlation: Reactive (Inflammatory) Lymphadenitis / 466
Throughout history, it has been noted that people who recover from certain diseases such as chickenpox, measles, and mumps become resistant (i.e., immune) to the same disease. Another longstanding observation is that immunity is specific—that is, immunity to chickenpox does not prevent infection with measles. We also now recognize that the immune system can react against itself, causing autoimmune diseases such as lupus erythematosus, autoimmune hemolytic anemia, some forms of diabetes mellitus, and autoimmune thyroiditis (Hashimoto’s thyroiditis).
The lymphatic system consists of groups of cells, tissues, and organs that monitor body surfaces and internal fluid compartments and react to the presence of potentially harmful substances. Lymphocytes are the definitive cell type of the lymphatic system and the effector cells in the response of the immune system to harmful substances. Included in this system are the diffuse lymphatic tissue, lymphatic nodules, lymph nodes, spleen, bone marrow, and thymus (Fig. 14.1). The various lymphatic organs and lymphatic tissues are often collectively referred to as the immune system.
Lymphatic vessels connect parts of the system to the blood vascular system.
Lymphatic tissues serve as sites where lymphocytes proliferate, differentiate, and mature. In addition, in the thymus, bone marrow, and gut-associated lymphatic tissue (GALT), lymphocytes are “educated” to recognize and destroy specific antigens. These are now immunocompetent cells that can distinguish between “self” (molecules normally present within an organism) and “nonself” (foreign molecules—i.e., those not normally present).
An antigen is any substance that can induce a specific immune response.
The body is constantly exposed to pathogenic (diseasecausing) organisms and hazardous substances from the external environment (infectious microorganisms, toxins, and foreign cells and tissues). In addition, changes may occur in cells (such as transformation of normal cells into cancerous cells) that give them characteristics of foreign cells. An immune response is generated against a specific antigen, which can be a soluble substance (e.g., a foreign protein, polysaccharide, or toxin) or an infectious organism, foreign tissue, or transformed tissue. Most antigens must be “processed” by cells of the immune system before other cells can mount the immune response.