Anatomy & Physiology

Anatomy & Physiology Study Guide

21.7 The Lymphatic System and Body Defenses The human body has multiple defense mechanisms that together provide resistance—the ability to fight infection, illness, and disease. Body defenses can be sorted into two general categories: Nonspecific defenses do not distinguish one type of threat from another. Their response is the same, regardless of the type of invading agent. Present at birth, these defenses include physical barriers, fever, inflammation, phagocytic cells, immunological surveillance, complement, and interferons. They provide a defensive capability known as nonspecific resistance. Specific defenses protect against particular threats. For example, a specific defense may protect against infection by one type of bacterium but be ineffective against other bacteria and viruses. Many specific defenses develop after birth as a result of accidental or deliberate exposure to environmental hazards. The body’s specific defenses produce a state of protection known as immunity, or specific resistance. Non-Specific Defenses Nonspecific defenses prevent the approach, deny the entry, or limit the spread of microorganisms or other environmental hazards. Physical barriers keep hazardous organisms and materials outside the body. For example, a mosquito that lands on your head may be unable to reach the surface of the scalp if you have a full head of hair. Phagocytes engulf pathogens and cell debris. Examples of phagocytes are the macrophages of peripheral tissues and the microphages of blood. Immunological surveillance uses NK cells to destroy abnormal cells. Interferons are chemical messengers that coordinate the defenses against viral infections. Complement is a system of circulating proteins that assists antibodies in the destruction of pathogens. The inflammatory response is a localized, tissue- level response that tends to limit the spread of an injury or infection. Fever is an elevation of body temperature that accelerates tissue metabolism and the activity of defenses. Physical Barriers An antigenic compound or a pathogen must enter body tissues to cause disease. This act requires crossing an epithelium—either at the skin or across a mucous membrane. The epithelial covering of the skin has multiple layers, a keratin coating, and a network of desmosomes that lock adjacent cells together. These barriers provide very effective protection for underlying tissues. Phagocytes Phagocytes perform janitorial and police services in peripheral tissues, removing cellular debris and responding to invasion by foreign compounds or pathogens. Phagocytes represent the “first line of cellular defense” against pathogenic invasion. Many phagocytes attack and remove microorganisms even before lymphocytes detect their presence. The human body has two general classes of phagocytic cells: microphages and macrophages. Microphages are the neutrophils and eosinophils that normally circulate in the blood. These phagocytic cells leave the bloodstream and enter peripheral tissues that have been subjected to injury or infection. Macrophages are large, actively phagocytic cells. Typically, macrophages are either freely mobile or fixed in position, and as a result, they are usually classified as fixed macrophages or free macrophages . An activated macrophage may respond to a pathogen in several ways: engulf it and destroy it with lysosomal enzymes, bind to or remove a pathogen from the interstitial fluid and get assistance from other cells to destroy it, or

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