Systemic Function of the Immune System

The many different cells, organs, and tissues of the immune system are dispersed throughout the body, yet the various components communicate and collaborate to produce an ef fective response to an infection. An infection that begins in one area of the body initiates processes that eventually involve cells, organs, and tissues distant from the site of pathogen invasion. Consider what happens when the skin is broken, allowing bacteria to enter the body and initiate infection.

The tissue damage associated with the injury and infection results in an inflammatory response that causes increased blood flow, vasodilation, and an increase in capillary permeability. Chemotactic signals are generated that can cause phagocytes and lymphocytes to leave the blood stream and enter the affected area. Factors generated during these early stages of the infection stimulate the capacity of the adaptive immune system to respond. Langerhans cells (dendritic cells found throughout the epithelial layers of the skin and the respiratory, gastrointestinal, urinary, and genital tracts) can capture antigens from invading pathogens and migrate into a nearby lymphatic vessel, where the flow of lymph carries them to nearby lymph nodes. In the lymph nodes these class II MHC-bearing cells can become members of the interdigitating dendritic-cell population and initiate adaptive immune responses by presenting antigen to TH cells. The recognition of antigen by TH cells can have important consequences, including the activation and proliferation of Th cells within the node as the TH cells recognize the antigen, and the secretion by the activated T cells of factors that support T-cell-dependent antibody production by B cells that may already have been activated by antigen delivered to the lymph node by lymph. The antigen-stimulated TH cells also release chemotactic factors that cause lymphocytes to leave the blood circulation and enter the lymph node through the endothelium of the postcapillary venules. Lymphocytes that respond to the antigen are retained in the lymph node for 48 hours or so as they undergo activation and proliferation before their release via the node's efferent lymphatic vessel.

Once in the lymph, the newly released activated lymphocytes can enter the bloodstream via the subclavian vein. Eventually, the circulation carries them to blood vessels near the site of the infection, where the inflammatory process makes the vascular endothelium of the nearby blood vessels more adherent for activated T cells and other leukocytes (see Chapter 15). Chemotactic factors that attract lymphocytes, macrophages, and neutrophils are also generated during the inflammatory process, promoting leukocyte adherence to nearby vascular epithelium and leading leukocytes to the site of the infection. Later in the course of the response, pathogen-specific antibodies produced in the node are also carried to the bloodstream. Inflammation aids the delivery of the anti-pathogen antibody by promoting increased vascular permeability, which increases the flow of antibody-containing plasma from the blood circulation to inflamed tissue. The result of this network of interactions among diffusible molecules, cells, organs, the lymphatic system, and the circulatory system is an effective and focused immune response to an infection.

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