The Lymphatic System

35.3 Lymphatic vessels resemble veins in that they have thin walls, and they have valves to prevent backflow (Applegate, 20000, 289).

35.4 Lymphatic capillaries differ from blood capillaries in that they begin in the tissue spaces as blind-ended sacs, whereas blood capillaries are extensions of arterioles (Applegate, 2000, 288). The endothelium of the lymphatic capillaries is also different in that the simple squamous cells overlap to form a one-way valve which lets tissue fluid in but does not let anything out. This differs from blood capillaries in which there is two-way exchange across the endothelium.

35.5 The function of the lymphatic vessels is to return excess tissue fluid to the circulation (Applegate, 2000, 287). They are also responsible for the absorption of fats and fat-soluble vitamins from the digestive system. This occurs in special lymphatic capillaries known as lacteals, which are found in the center of the villi in the small intestine.

35.6 Lymph fluid has a similar composition to blood plasma (Applegate, 2000, 287)). As blood passes through the capillaries, fluids pass

. . .
irected against virus-infected cells, cancer cells, foreign tissue cells (in transplants), fungi, and protozoan parasites. Killer T cells directly destroy cells containing the specific antigen. Helper T cells secrete substances that stimulate B cells and promote the immune response. Suppressor T cells inhibit B cells and the immune response. 35.17 Memory T cells remember the specific antigen they have encountered and this allows them to initiate a faster and more intense response when exposed to the antigen again in the future (Applegate, 20000, 296-297). When B cells are activated by an antigen, they respond by dividing and producing two clones of cells: one clone consists of plasma cells which rapidly produce large amounts of antibody against the antigen; the second clone consists of memory B cells which lie dormant until the antigen is encountered again. On a second exposure to the antigen, the memory B cells provide a rapid and intense response against the antigen. 35.18 The specificity of the immune system lies in the fact that T and B cells in the immune system only have receptors for one antigen on their cell surface, i.e. they can only react with one specific antigen (Applegate, 2000, 296). 35.19 Duri
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Approximate Word count = 1980
Approximate Pages = 8 (250 words per page)

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