Immune System Defense Mechanisms

The lung is similar to all other organs by containing lymphocytes (T and B cells) in the interstitium. These defense cells originate from the bone marrow and lymph nodes and respond to foreign invaders with cellular (acquired antibody) mechanisms. Dendritic cells present antigens to the lymphocytes. Considering that up to 1010 antigenic particles may reach the alveoli every day, the challenge for the pulmonary immune system is to process this foreign material and not overamplify an inflammatory response. Basic immune mechanisms in the lungs are similar to the rest of the body and are not covered here.

Lymphatics are the main pathway for removal of immune cells that have already responded to foreign substances or cells in the lungs. The lymphatic drainage also removes excess fluid filtration from the pulmonary capillaries as described earlier. Pulmonary lymphatics start as blind end vessels in the acini, where they collect fluid and lymphocytes through a leaky endothelium. Lymphatics do not occur at the level of the alveoli. Lymph flow is always directed out of the lungs by numerous valves in a network of vessels that generally follows the large airways and blood vessels. Lymph nodes occur along this network and function as biological filters. Macrophages and immune defense cells collect in lymph nodes and present antigens to immu-nocompetent cells in the nodes. This programs new immune system defenses for future invasions. Ultimately, pulmonary lymph returns to the venous system near the junction of the subclavian and jugular veins.

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