After traversing the epithelium and its basement membrane to reach the subepithelial tissues, virions are immediately exposed to tissue macrophages and can enter the lymphatics that form a network beneath the skin and all mucosal epithelia (Fig. 6-3). Virions that enter lymphatics are carried to local lymph nodes. As they enter, they are exposed to macrophages lining margin-
al sinuses and may be engulfed. Virions may be inactivated and processed and their component antigens presented by macrophages and dendritic cells to adjacent lymphocytes in such a way that an immune response is initiated (see Chapter 8). Some viruses, however, replicate in cells of the monocyte/macrophage lineage; others infect lymphocytes. Some virions may pass straight through Jymph nodes to enter the bloodstream. Monocytes and lymphocytes circulate through the body, and there is also a constant movement of lymphocytes directly from the blood into the lymph nodes, and in the opposite direction.
There is often a local inflammatory response, the extent of which depends on the extent of tissue damage. Local blood vessels are dilated and rendered more permeable, so that monocytes and lymphocytes, lymphokines, immunoglobulins, and complement components can be delivered directly to the site of infection, wilh a consequent increase in host resistance, especially after the immune response has been initiated
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