The Lymphatic System

Excess [1] in the body's tissues enters microscopic, dead-end lymphatic vessels called [2] and becomes [3]. This fluid flows through larger and larger vessels, finally reaching the two largest vessels of the lymphatic system in the thoracic cavity. One of these, the [4], receives fluid from the right arm and right side of the head and thorax. The other, the [5], begins at a collecting sac in the abdomen called the [6], which receives lymph from all parts of the body inferior to the diaphragm. Ducts 4 and 5 them empty the lymph into the bloodstream at the [7] veins, returning the excess fluid (1) to the bloodstream.

Lymphatic tissue of the mucous membranes is called MALT, or [8] lymphatic tissue. In some places, it forms dense ovoid masses of lymphocytes called [9]. Along the course of the lymphatic vessels, there are many small, often bean-shaped organs called [10], which filter the lymph and respond to bacteria or other pathogens in it. These are the only lymphatic organs to have both [11] and [12] lymphatic vessels. The [13] are lymphatic organs that encircle the openings into the pharynx and respond to inhaled and ingested pathogens. The [14] is a lymphatic organ in the mediastinum that houses developing lymphocytes and secretes hormones that later regulate their activity. It is very large in newborn infants, but undergoes shrinkage, or [15], after childhood and is very small in adults. The [16], an organ in the upper left abdominal quadrant, has red pulp in which [17] are stored and [18] composed mainly of lymphocytes and macrophages. It is not only a source of immune cells, but an emergency reserve of blood cells and is nicknamed the [19] because old RBCs die and are disposed of here.

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Nonspecific Resistance

The tough epidermal protein [20] makes the skin an effective barrier to most pathogens. If they should get through it, they often become trapped in the tissue [21], which has a viscous consistency due to a glycosaminoglycan called [22]. Body secretion such as tears and saliva contain a surfactant called [23] that destroys many bacteria. The leukocytes that respond most actively to bacteria are the [24]. In addition to phagocytizing them, they can undergo a reaction called a [25], which creates a chemical [26] zone of strong oxidizing agents. Other WBCs, the [27], phagocytize antigen-antibody complexes and allergens. A class of lymphocytes called [28] destroy host (one's own) cells that have become cancerous or infected with viruses. All of the phagocytic cells except WBCs are called [29].

[30] is a response to tissue injury characterized by five cardinal signs, including [31], [32], swelling, pain, and functional loss. The pain is caused by the swelling and by chemicals such as [33], released by basophils and mast cells. Swelling results in part from [34], the increased blood flow to an inflamed tissue. When WBCs arrive at a site of tissue injury, they exhibit [35], or adhesion to the capillary walls; [36], or migration through the wall into the connective tissues; and [37], or attraction by inflammatory chemicals. In tissue repair, a growth factor derived from [38] stimulates mitosis and regrowth.

[39] is a protein secreted by cells infected with viruses. It signals neighboring cells to mount a defense before the virus attacks them. Bacteria and other foreign cells are often destroyed by a system of ß globulins called [40] in the blood. These proteins can promote inflammation; cause [41], or the rupture of foreign cells; or coat bacteria and make them easier to phagocytize, a process called [42]. Fever is another nonspecific defense, triggered by secretions called [43] that may stimulate the hypothalamus to raise the body temperature set point. During the [44] phase, the body temperature rises. During [45], the phase of stable elevated temperature, the high temperature interferes with the reproduction of pathogens. [46] is the phase in which the body loses heat and the temperature returns to normal.

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General Aspects of Specific Immunity

Unlike nonspecific defense, immunity is characterized by [47] and [48]–a selective response to a particular pathogen and a stronger reaction when the body is later reexposed to the same pathogen. It is divided into two forms–[49] immunity, which is mediated by circulating antibodies, and [50] immunity, in which lymphocytes directly attack foreign cells. Both types are triggered by foreign chemicals called [51] or by smaller molecules called [52] that can bind to one's own molecules and make them "look" foreign. In response to such agents, the body often makes g globulins called [53] that are either bound to the lymphocyte surface or secreted into the body fluids. These have a T or Y shape with a/an [54] site at the tip of each arm.

T lymphocytes (T cells) are named for the fact that they temporarily colonize the [55]. During their development in this site, they produce 10,000 to 100,000 surface receptors for antigens. Once these are in place, a T cell is said to be [56] because it is able to respond to an antigen. In the process of development, T cells programmed to respond to the body's own antigens are destroyed in a process called [57], leaving the body in a state of [58], in which it will attack only foreign antigens. Lymphocytes that do not develop in the 55, but go on to later produce circulating antibodies, are called [59]. They, as well as macrophages, also function as APCs, or [60] cells, which display bits of foreign antigen to activate an immune response. The activities of immune cells are coordinated by chemical signals called [61].

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Humoral Immunity

Each form of immunity can be divided into three phases: [62], [63], and [64]. In humoral immunity, the first phase begins when an antigen binds to several receptor sites on a B cell, a process called [65]. The B cells engulfs and digests the antigen, then displays portions of it attached to [66] at the cell surface. For the process to continue, a [67] cell must bind to this site and secrete lymphokines that stimulate the B cell. The B cell then multiplies–a process called [68]–and many of its daughter cells become antibody-secreting [69]. Antibodies act on pathogens in multiple ways that minimize their potential to cause disease. In [70], they cover a part of the antigen molecule that makes it pathogenic. In [71], they activate the binding of complement proteins to a foreign cell. In [72], they clump antigens together, forming antigen-antibody complexes which may then [73] from the body fluids and be phagocytized. Some B cells do not differentiate into plasma cells but become [74] cells, which mount a very quick [75] immune response if one is later reexposed to the same antigen.

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Cellular Immunity

In cellular immunity, T cells respond to antigens displayed by MHC-I and MHC-II proteins. An MHC-I protein displaying an antigen stimulates [76] T cells to attack diseased cells of the host body. While MHC-I proteins are found on all cells of the body, MHC-II proteins are found only on [77] and stimulate only [78] T cells. Cytotoxic T cells, the only ones that directly attack diseased or foreign cells, sometimes secrete a protein called [79] that opens a hole in the enemy cell. In other cases, they secrete [80], which destroys the enemy cell's DNA. As a pathogen is defeated, [81] T cells begin to inhibit B cells and other T cells, and the immune response subsides. Memory T cells remain and produce a quick [82] response if the person is reexposed to the same pathogen, similar to the secondary response of humoral immunity.

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Immune System Disorders

Allergy, or [83], is an intense immune reaction against antigens that most people tolerate. Antigens that trigger such a response are called [84]. Most allergies are in the type I, or [85], category, so called because the response begins and ends very quickly. This category includes [86], a reaction in which the antigen caps IgE molecules on basophils and mast cells and triggers the release of histamine and other inflammatory chemicals, causing local edema, congestion, itching, and so forth. In [87], this reaction occurs in the respiratory tract, where it causes breathing difficulty and sometimes fatal suffocation. If an antigen is introduced into the blood stream of a hypersensitive person, it can trigger a life-threatening, widespread reaction called [88]. Types II and III reactions are called [89] because they have a slower onset and longer course. They include transfusion reactions against mismatched blood and the immune reaction of systemic lupus erythematosus. [90] diseases occur when the immune system fails to distinguish host antigens from foreign antigens and thus lacks self-tolerance. This occurs, for example, in rheumatic fever and insulin-dependent [91]. By contrast, [92] diseases such as AIDS and SCID occur when the immune system is insufficiently responsive because of a deficiency of T cells, B cells, or both.

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