notes21 - 21. The Immune System: Innate and Adaptive Body...

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21. The Immune System: Innate and Adaptive Body Defenses I. Introduction Mechanisms your body employs to defend against pathogens and toxins make up your body’s immune system. This system differs from other systems studied in the class in that it is a system composed primarily of individual cells spread throughout the body, rather than a discrete system of organs. However, the cells of the immune system do work in a cooperative manner to provide a clear function for the body. Defense is provided by the immune system in two ways: (1) The innate (or nonspecific ) system attempts to protect the body from all invaders. The first line of defense in the innate system is provided by the skin and mucosae, which act as physical barriers to invasion. The second line of defense includes phagocytic cells and antimicrobial proteins, which attempt to contain the spread of any invaders that get past the first line of defense. The innate system provides a generic but immediate defense against toxins and pathogens. (2) The adaptive (or specific ) system can mount specific attacks against specific invaders. This system can provide a more effective defense against a specific invader, but it takes more time for the adaptive system to coordinate an attack. Although the innate and adaptive systems play different roles in defending the body, they generally act together in fighting an infection. INNATE DEFENSES II. Surface Barriers: Skin and Mucosae The thick, keratinized stratum corneum provides a nearly impenetrable barrier to microorganisms that might try to enter the body through intact skin. Mucous membranes also provide an effective barrier against most potential invaders. Various secretions help prevent entry into the body: 1. Fluids secreted onto the skin ( e.g., sweat and sebum) contain chemicals that inhibit growth of microorganisms. 2. The stomach secretes hydrochloric acid and proteases that kill microorganisms in the food we eat. 3. Saliva and tears contain lysozyme to kill bacteria residing in the mouth or on the surfaces of the eye. 4. Mucous membranes produce mucus to trap microorganisms. Cilia lining internal passages sweep mucus out of the body or into the digestive tract. Mucus also contains lysozyme. III. Internal Defenses: Cells and Chemicals Phagocytes Phagocytic cells patrol the body’s tissues in search of invaders that have breached the surface barriers. The chief phagocytes are macrophages , which are monocytes that have left the bloodstream and entered the tissues. Neutrophils are another type of cell that can phagocytize foreign particles. Eosinophils are sometimes phagocytic, but they are more important in defending against invaders too large for phagocytosis, such as parasitic worms. The mechanism of phagocytosis is shown in Fig. 21.2.
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notes21 - 21. The Immune System: Innate and Adaptive Body...

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