Host defense, dendritic cells and the human lung

Host defense mechanisms protect the body against microorganisms and other foreign structures. These mechanisms can be divided in nonspecific, or innate, and specific, or acquired, immunity. In both branches of immunity the several types of leukocytes (white blood cells) play a dominant role. Nonspecific defense comprises the natural barriers that protect against invading microorganisms, the complement system, and various types of white blood cells including granulocytes, monocytes, macrophages and natural killer cells that are able to neutralize microorganisms and foreign material by phagocytosis and/or killing. The natural barriers consist of the epithelial surfaces of the body, such as the skin and the bronchial epithelium. In the lung, junctional complexes between bronchial epithelial cells physically prevent the invasion of microorganisms. In addition, microorganisms and other particles are removed from the respiratory tract by the "tapis roulant": the mucus film propelled towards the oropharyngeal cavity by the cilia of the epithelial cells. The mucus not only serves as vehicle for particle transport, but also contains antibacterial and antiviral proteins. Specific immunity involves the recognition of foreign structures (antigens), the discrimination between self and non-self, and the generation of immunologic memory. A crucial step in the initiation of an immune response is the presentation of antigens to and the stimulation of T cells. Cells capable of antigen-presentation and the stimulation of T cells are called antigenpresenting cells (APC) and comprise monocytes, macrophages, B cells and in particular dendritic cells (DC). Recognition of the presented antigen by the T cell and the generation of costimulatory signals by the APC result in proliferation, cytokine production and a change in marker expression of the T cell. The T cells thereby regulate the activation of several effector cells such as 8 cells (that produce antibodies), and macrophages. These effector mechanisms are directed to eliminate the antigen.

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