Lymphatic System 2015
Emma Jakoi, Ph.D.
The ability of the body to protect itself from pathogens, such as viruses and bacteria, is known as immunity. In the lectures on blood, you learned that phagocytic cells can mount a general pro-inflammatory response to pathogens, called innate immunity. The primary cells of innate immunity are macrophages and neutrophils and their weapons are oxidizing agents and phagocytosis. Some of these cells (i.e., macrophages) also signal the presence of foreign substances to lymphocytes, the mediators of acquired immunity. The small lymphocytes that recognize foreign substance (antigen) are called immunocompetent cells. They neutralize (or destroy) the pathogens and provide a memory of these encounters when re-exposed to the same pathogen. Their weapons include antibodies, cytokines, and cytolytic activities, and their actions are called the immune response.
There are millions of different types of lymphocytes each programmed to combat a different antigen. They serve three major functions:
1. Protection from disease causing invaders or pathogens
2. Removal of dead or damaged tissue and cells
3. Recognition and removal of abnormal cells
Once the foreign substance is removed, then a state of equilibrium (immune homeostasis) is regained by activation of negative feedback mechanisms which terminate the immune response and return the immune system to a basal state. These regulatory mechanisms also maintain unresponsiveness (tolerance) to ourselves (i.e., to self-antigens, molecules originating within the body).
The parenchyma (dominant cell type) of the organs and tissues of the immune system are lymphocytes. Unlike the parenchyma of other organs such as the heart and lung, small lymphocytes are migratory cells. They transiently lurk and often accomplish their jobs within tissues and organs distributed diffusely throughout the body (Fig 1). Because small lymphocytes are the dominant cellular population in these tissues and organs, the immune system is also called the lymphatic system (or lymphoid system).
Lymphatic tissue lies within the connective tissue underlying epithelium and in non-lymphatic and lymphatic organs. It varies in appearance and can be classified by increasing structural and functional complexity from the simplest random array of cells (called diffuse tissue) to more organized aggregates (called nodular tissue). The organs of the lymphatic system also vary in complexity and include the partially encapsulated tonsils as well as the encapsulated thymus, lymph node, and spleen. Lymphatic organs may contain diffuse and/or nodular tissue.
Small lymphocytes migrate into and out of lymphatic tissues and organs over 4 to 6 hours as part of a general surveillance. However, once a foreign antigen is encountered, recruitment of other small lymphocytes to that region occurs within minutes from the blood and lymphatic circulations.
Figure 1. Tissues and organs of the lymphatic system
In the next two lectures, we will deal with the cells, tissues, and organs that comprise this immune system. To gain an appreciation for how these organs function and their structure, we will consider briefly the effector cells and their interactions in the immune response. The details of acquired immunity will be presented in the Immunology component of Body and Disease.
The first lecture covers the thymus and lymph nodes; the second covers the spleen
Constituents and function in immune surveillance
Lymphocytes are classified as one of two basic types established by colonization either from the thymus (T lymphocytes) or from the bone marrow (B lymphocytes) (Fig. 2). They are morphologically identical but can be distinguished by their functions and surface markers.