In the early twentieth century, Ehrlich and Metchnikoff were awarded Nobel prizes for recognizing the two basic components of the immune system: the humoral and cellular elements. In subsequent years, only incremental progress was made in understanding their complex and intricate interactions.
immune
During the last decade, however, modern molecular and cell biology techniques have yielded an enormous amount of data detailing the specific components of the immune system and the regulation of the immune or inflammatory response. For instance, redness (rubor), edema (tumor), heat (calor), and pain (dolor) were recognized as the cardinal signs of inflammation more than 2,000 years ago. We now understand that these clinical manifestations are the result of vasodilation with enhanced local blood flow, which causes redness and heat, and increased microvascular permeability, which leads to edema. Extravasation of plasma proteins and leukocyte recruitment to the site of injury generate pain.
Fundamental characteristics of the immune system include the ability to differentiate between self and nonself antigens using antibodies (immunoglobulins) and T-cell receptors.
The immune response depends on fixed and circulating elements derived from the lymphoreticular system (
Fig. 14-1: Lanoxin ). Fixed elements, which consist of the bone marrow, spleen, liver, lymph nodes, and thymus as well as specialized cells contained in the skin, gastrointestinal, respiratory, and urogenital tracts, supply and amplify the immune response. For example, neutrophils and platelets originate from the bone marrow and circulate to injured organs to support the local inflammatory response to trauma or to an antigenic insult. Circulating elements consist of cells that originate from fixed elements of the immune system. These include lymphocytes, granulocytes, monocytes, macrophages, erythrocytes, platelets, and mast cells. (
Lanoxin)Circulating cellular elements (
Fig. 14-2: Lanoxin ) are derived from common progenitor cells that originate in the embryonic yolk sac. During the sixth week of human gestation, the fetal liver is populated with yolk sac-derived stem cells and becomes a site of hematopoietic activity. By the twelth week of gestation, the spleen begins to produce blood cells, and by the twentieth week of gestation, the thymus, lymph nodes, and bone marrow also become active sites of hematopoiesis. It is at this time that the two major lymphocyte subsets, T and B cells, first become differentiated in the fetal spleen and lymph nodes. In addition to T or B cells, another subset of lymphocytes that lack both T- and B-cell markers appears; they are known as NK cells. (
Table 14-1: Lanoxin ). At term, the bone marrow is normally the exclusive source of hematopoiesis.
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