Revision:The Defences of the Human Body

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The Skin

In Humans the skin has two major layers. The outer, relatively thin epidermis is composed of closely packed cells with little intercellular material; it provides the barrier against attack by chemicals, radiation, or microbes. The underlying dermis is thicker and tougher, and extra cellular materials manufactured by scattered cells form its bulk. One of its major functions is physical protection. The sensory functions of skin are shared by both epidermis and dermis.

Immunization

Immunization is the process of acquiring or inducing resistance to infection in a human. The fundamental purpose of immunization is to initiate the production of antibodies. The antibody-antigen interaction takes place in the blood, in the extra cellular fluid, at mucosal surfaces, or in viruses, before initial intracellular penetration or before the virus moves from the entry site to the target tissue. This interaction causes either the modulation or the destruction of the antigen. Immunization may also initiate the mobilization of lymphocytes and macrophages in a process called cellular immune response.

Vaccination

Vaccination is suspension of either weakened or killed a microorganism that is capable of causing antibody production against an infectious microorganism when artificially introduced into the body, thereby conferring immunity from a subsequent infection of that microorganism. A vaccine can also consist of the weakened toxins produced by microorganisms. Once stimulated by a vaccine, the antibody-producing cells of the body remain sensitized to the infectious agent and respond to reinfection by producing more antibodies, thus reinstituting the immune response.

The Lymph System

Lymph nodes expose microorganisms and other substances circulating in the lymph to infection-fighting phagocyte cells and lymphocytes. Great numbers of lymph nodes are distributed along the lymphatic vessels. Lymph channels leading to the nodes conduct lymph slowly into and out of the node. As the lymph is filtered through the node, it is exposed to many phagocyte cells, cells that can engulf microorganisms or other foreign materials. Foreign materials in the lymph are particularly exposed to lymphocytes, which attack the invaders directly or synthesize antibodies against them. Another function of the node is to add lymphocytes and antibodies to the lymph as it passes through, ensuring that the capacity for immune response pervades the entire lymphatic system.

Blood Clots (Hemostasis)

The evolution of high-pressure blood circulation in vertebrates has brought with it the risk of bleeding after injury to tissues. Mechanisms to prevent bleeding (i.e., hemostatic mechanisms) are essential to maintain the closed blood-circulatory system. Normal homeostasis is the responsibility of a complex system of three individual components: blood cells (platelets), cells that line the blood vessels (endothelial cells), and blood proteins (blood-clotting proteins). The blood platelet is a no nucleated cell that circulates in the blood in an inactive, resting form. Endothelial cells line the wall of the blood vessel and inhibit blood from clotting on the vessel wall under normal conditions. Blood-clotting proteins circulate in the blood plasma in an inactive form, poised to participate in blood coagulation upon tissue injury. Blood-clotting proteins generate thrombin, an enzyme that converts fibrinogen to fibrin, and a reaction that leads to the formation of a fibrin clot.

Body Secretions

Exocrine glands are any gland that secretes substances through a duct that in turn opens on an internal body cavity (such as the mouth or gut) or an external surface of the body (the skin or eye). It contrasts with an endocrine gland, which secretes its products (hormones) directly into the circulatory system. Saliva, sweat, and tears are products of exocrine glands.  On the skin there are oil glands, which lubricate and protect against bacteria, in the mouth mucus and serous glands produce saliva to protect the mouth and throat and aid digestion.  The skin stops the body from over heating by secreting sweat, which is evaporated and digested by local bacteria on the skin surface and the lachrymal gland secretes tears into the eye when the cornea is irritated.

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