37. Question - Name two general classes of lymphocytes. Describe their morphology as they usually appear in the circulation. What are their concentrations in peripheral blood? What are their sources?
Two general classes of lymphocytes are B- and T-cells. Lymphocytes make up about 30% of the circulating leukocytes and of those the T-cells make up about 90%. Most of the lymphocytes in the circulating blood have a compact nucleus and a thin rim of cytoplasm. A few are medium or large lymphocytes. These are activated cells that will proliferate or differentiate to become small memory cells, plasma cells or other active immune cells.
B-cells derive from and undergo their definitive differentiation in the bone marrow (bursa of Fabricius in birds). T-cell precursors come form the bone marrow and settle in the thymus where they differentiate into mature T-cells. But since most T-cells would react adversely against body tissues they die by apoptosis in the thymus.
38. Question - Using diagrams outline the process of lymphocyte activation and its consequences.
39. Question - Describe two different functions of macrophages (histiocytes). What names have been given to the system of macrophages throughout the body?
The primary function of macrophages is to phagocytose and digest tissue fragments especially apoptotic bodies. They also digest tissue fragments resulting from tissue injury and death. Certain forms of macrophages, antigen presenting cells, will as a result of these processes combine antigen fragments (obtained from the debris that has been digested) with major histocompatability proteins and present these on their surface to the immune system.
Reticulo-endothelial system is the classical name for the system of macrophages but that name was used in the belief that certain endothelial cells had special phagocytic activity. The name for this system that histologists have been promoting for many years is mononuclear phagocytic system. It includes the microglia of the central nervous system.
40. Question - Describe three different morphological manifestations of lymphoid tissue associated with mucous membranes. What is a lymph nodule?
Mucous membranes are wet and are particularly susceptible to the invasion of foreign organisms. Therefore lymphocytes are often found in high concentration just under the epithelium. This is called diffuse lymphoid tissue. Some of the lymphocytes invade the epithelium and these are called epithelial lymphocytes.
Solitary lymph nodules are found in similar locations and they consist of a tightly organized cluster of mostly B-lymphocytes. When it is active there is a central region of large cells that is called the reaction or germinal center. Nodules without a germinal center are called primary nodules (follicles) and those with a germinal center are called secondary nodules (follicles).
The tonsils, Peyer's patches and the appendix are locations where aggregates of nodules occur.
41. Question - Using diagrams describe the locations and morphological differences between the tonsils.
42. Question - What are epithelial lymphocytes? Where are they found and where are they most numerous? What are "M" cells? Where are they found and what is their function?
Lymphocytes often cross the basement membrane of epithelia of mucous membranes to wander among the epithelial cells. These are mostly T-cells and they are called epithelial lymphocytes. They are most numerous in epithelia over aggregates of lymph nodules.
Some columnar cells in simple columnar epithelium over lymph nodules in the intestinal tract are specialized for antigen processing. They present these antigens to the underlying lymphoid tissue. M cells have a specialized surface so that instead of a striated border of microvilli they have short microfolds. "M" stands for microfolds. These cells are especially abundant over nodules of Peyer's patch.
43. Question - What is lymph? What is its source and destination? Name and identify the channels and structures through which it passes.
A filtrate of plasma leaves capillaries at their arterial end and a portion returns to the blood at their venous end. The excess interstitial fluid is taken up by capillaries and blind-ended lacteals of the lymph system. This lymph fluid may contain wandering lymphocytes but few other cells.
Lymph capillaries join to form larger vessels and these form afferent vessels that empty into lymph nodes. Efferent vessels that flow out of nodes may lead to other nodes before continuing in the thoracic duct and other lymphatic trunks that empty into veins.
44. Question - Diagram a lymph node naming its various parts.
45.Question - Diagram two paths followed by T lymphocytes as they leave and reenter the blood.
Lymphocytes, especially T-cells, leave the blood through capillaries and venules in the peripheral circulation and through postcapillary venules in lymph nodes. In each of those sites they enter the lymph and then follow the lymph back to the heart.
46. Question - Diagram the spleen naming its various parts. Identify two functions of the spleen.
The spleen filters blood removing old erythrocytes and foreign antigens to which an immune response can be mounted. Blood passes through the pulp cords to the sinuses. On the way it encounters macrophages.
47. Question - Diagram the thymus naming its various parts. What is the function of the thymus? Describe its stroma indicating its origin.
The thymus is called a primary lymphoid organ because it is here that T lymphocytes differentiate from precursor cells that come from the bone marrow. Rather than reticular connective tissue as is found in other lymphoid organs the thymus has an epithelial stroma. This epithelium pinches off of the third pharyngeal pouch and migrates to its destination where the precursor cells invade the epithelium.
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Last Revised: Thu, Jan 25, 2007
