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<p><strong>Immunology</strong> is a broad branch of <a title="Biomedical science" href="http://en.wikipedia.org/wiki/Biomedical_science">biomedical</a> <a title="Science" href="http://en.wikipedia.org/wiki/Science">science</a> that covers the study of all aspects of the <a title="Immune system" href="http://en.wikipedia.org/wiki/Immune_system">immune system</a> in all <a title="Organism" href="http://en.wikipedia.org/wiki/Organism">organisms</a>. It deals with, among other things, the <a title="Physiology" href="http://en.wikipedia.org/wiki/Physiology">physiological</a> functioning of the immune system in states of both health and disease; malfunctions of the immune system in immunological disorders (<a title="Autoimmune diseases" href="http://en.wikipedia.org/wiki/Autoimmune_diseases">autoimmune diseases</a>, <a title="Hypersensitivity" href="http://en.wikipedia.org/wiki/Hypersensitivity">hypersensitivities</a>, <a title="Immune deficiency" href="http://en.wikipedia.org/wiki/Immune_deficiency">immune deficiency</a>, <a title="Transplant" href="http://en.wikipedia.org/wiki/Transplant">allograft</a> rejection); the physical, chemical and physiological characteristics of the components of the immune system <a title="In vitro" href="http://en.wikipedia.org/wiki/In_vitro">in vitro</a>, <a title="In situ" href="http://en.wikipedia.org/wiki/In_situ">in situ</a>, and <a title="In vivo" href="http://en.wikipedia.org/wiki/In_vivo">in vivo</a>. Immunology has various applications in several disciplines of science, and as such is further divided.</p> 

<p>Even before the concept of <a title="Immunity (medical)" href="http://en.wikipedia.org/wiki/Immunity_%28medical%29">immunity</a> (from <em>immunis</em>, Latin for &quot;exempt&quot;) was developed, numerous early physicians characterised organs that would later prove to be part of the immune system. The key organs of the immune system are <a title="Thymus" href="http://en.wikipedia.org/wiki/Thymus">thymus</a>, <a title="Spleen" href="http://en.wikipedia.org/wiki/Spleen">spleen</a>, <a title="Bone marrow" href="http://en.wikipedia.org/wiki/Bone_marrow">bone marrow</a>, <a title="Lymphatic system" href="http://en.wikipedia.org/wiki/Lymphatic_system">lymph vessels</a>, <a title="Lymph node" href="http://en.wikipedia.org/wiki/Lymph_node">lymph nodes</a> and secondary lymphatic tissues such as <a title="Tonsil" href="http://en.wikipedia.org/wiki/Tonsil">tonsils</a>, <a title="Adenoid" href="http://en.wikipedia.org/wiki/Adenoid">adenoids</a>, and <a title="Skin" href="http://en.wikipedia.org/wiki/Skin">skin</a>. When health conditions warrant, immune system organs including the thymus, spleen, portions of bone marrow, lymph nodes and secondary lymphatic tissues can be <a title="Surgery" href="http://en.wikipedia.org/wiki/Surgery">surgically</a> excised for examination while patients are still alive.</p><p>Many components of the immune system are actually <a title="Cell (biology)" href="http://en.wikipedia.org/wiki/Cell_%28biology%29">cellular</a> in nature and not associated with any specific organ but rather are embedded or circulating in various <a title="Tissue (anatomy)" href="http://en.wikipedia.org/wiki/Tissue_%28anatomy%29">tissues</a> located throughout the body.</p>

<p>Classical immunology ties in with the fields of <a title="Epidemiology" href="http://en.wikipedia.org/wiki/Epidemiology">epidemiology</a> and <a title="Medicine" href="http://en.wikipedia.org/wiki/Medicine">medicine</a>. It studies the relationship between the body systems, <a title="Pathogen" href="http://en.wikipedia.org/wiki/Pathogen">pathogens</a>, and immunity. The earliest written mention of immunity can be traced back to the <a title="Pandemic" href="http://en.wikipedia.org/wiki/Pandemic">plague</a> of <a title="Athens" href="http://en.wikipedia.org/wiki/Athens">Athens</a> in <a title="430 BCE" href="http://en.wikipedia.org/wiki/430_BCE">430 BCE</a>. <a title="Thucydides" href="http://en.wikipedia.org/wiki/Thucydides">Thucydides</a> noted that people who had recovered from a previous bout of the disease could <a title="Nurse" href="http://en.wikipedia.org/wiki/Nurse">nurse</a> the sick without contracting the illness a second time. Many other ancient societies have references to this phenomenon, but it was not until the <a title="19th century" href="http://en.wikipedia.org/wiki/19th_century">19th</a> and <a title="20th century" href="http://en.wikipedia.org/wiki/20th_century">20th centuries</a> before the concept developed into scientific theory.</p><p>The study of the molecular and cellular components that comprise the immune system, including their function and interaction, is the central science of immunology. The immune system has been divided into a more primitive <a title="Innate immunity" href="http://en.wikipedia.org/wiki/Innate_immunity">innate immune system</a>, and <a title="Adaptive immunity" href="http://en.wikipedia.org/wiki/Adaptive_immunity">acquired or adaptive immune system</a> of vertebrates, the latter of which is further divided into <a title="Humoral immunity" href="http://en.wikipedia.org/wiki/Humoral_immunity">humoral</a> and <a title="Cell-mediated immunity" href="http://en.wikipedia.org/wiki/Cell-mediated_immunity">cellular components</a>.</p><p>The humoral (antibody) response is defined as the interaction between <a title="Antibody" href="http://en.wikipedia.org/wiki/Antibody">antibodies</a> and <a title="Antigen" href="http://en.wikipedia.org/wiki/Antigen">antigens</a>. Antibodies are specific proteins released from a certain class of immune cells (B lymphocytes). Antigens are defined as anything that elicits generation of antibodies, hence they are <strong>Anti</strong>body <strong>Gen</strong>erators. Immunology itself rests on an understanding of the properties of these two biological entities. However, equally important is the cellular response, which can not only kill infected cells in its own right, but is also crucial in controlling the antibody response. Put simply, both systems are highly interdependent.</p><p>In the <a title="21st century" href="http://en.wikipedia.org/wiki/21st_century">21st century</a>, immunology has broadened its horizons with much research being performed in the more specialized niches of immunology. This includes the immunological function of cells, organs and systems not normally associated with the immune system, as well as the function of the immune system outside classical models of immunity.</p>

<p>Clinical immunology is the study of <a title="Disease" href="http://en.wikipedia.org/wiki/Disease">diseases</a> caused by disorders of the immune system (failure, aberrant action, and malignant growth of the cellular elements of the system). It also involves diseases of other systems, where immune reactions play a part in the pathology and clinical features.</p><p>The diseases caused by disorders of the immune system fall into two broad categories: <a title="Immunodeficiency" href="http://en.wikipedia.org/wiki/Immunodeficiency">immunodeficiency</a>, in which parts of the immune system fail to provide an adequate response (examples include <a title="Chronic granulomatous disease" href="http://en.wikipedia.org/wiki/Chronic_granulomatous_disease">chronic granulomatous disease</a>), and <a title="Autoimmunity" href="http://en.wikipedia.org/wiki/Autoimmunity">autoimmunity</a>, in which the immune system attacks its own host's body (examples include <a title="Systemic lupus erythematosus" href="http://en.wikipedia.org/wiki/Systemic_lupus_erythematosus">systemic lupus erythematosus</a>, <a title="Rheumatoid arthritis" href="http://en.wikipedia.org/wiki/Rheumatoid_arthritis">rheumatoid arthritis</a>, <a title="Hashimoto's disease" href="http://en.wikipedia.org/wiki/Hashimoto%27s_disease">Hashimoto's disease</a> and <a title="Myasthenia gravis" href="http://en.wikipedia.org/wiki/Myasthenia_gravis">myasthenia gravis</a>). Other immune system disorders include different <a title="Hypersensitivity" href="http://en.wikipedia.org/wiki/Hypersensitivity">hypersensitivities</a>, in which the system responds inappropriately to harmless compounds (<a title="Asthma" href="http://en.wikipedia.org/wiki/Asthma">asthma</a> and <a title="Allergy" href="http://en.wikipedia.org/wiki/Allergy">allergies</a>) or responds too intensely.</p><p>The most well-known disease that affects the immune system itself is <a title="Acquired Immunodeficiency Syndrome" href="http://en.wikipedia.org/wiki/Acquired_Immunodeficiency_Syndrome">AIDS</a>, caused by <a title="HIV" href="http://en.wikipedia.org/wiki/HIV">HIV</a>. AIDS is an immunodeficiency characterized by the lack of CD4+ (&quot;helper&quot;) <a title="T cells" href="http://en.wikipedia.org/wiki/T_cells">T cells</a> and <a title="Macrophages" href="http://en.wikipedia.org/wiki/Macrophages">macrophages</a>, which are destroyed by HIV.</p><p>Clinical immunologists also study ways to prevent <a title="Transplant rejection" href="http://en.wikipedia.org/wiki/Transplant_rejection">transplant rejection</a>, in which the immune system attempts to destroy <a title="Allograft" href="http://en.wikipedia.org/wiki/Allograft">allografts</a> or <a title="Xenograft" href="http://en.wikipedia.org/wiki/Xenograft">xenografts</a>.</p>

<dl><dd><em>See main article <a title="Immunotherapy" href="http://en.wikipedia.org/wiki/Immunotherapy">Immunotherapy</a></em> </dd></dl><p>The use of immune system components to treat a disease or disorder is known as immunotherapy. Immunotherapy is most commonly used in the context of the treatment of <a title="Cancer" href="http://en.wikipedia.org/wiki/Cancer">cancers</a> together with <a title="Chemotherapy" href="http://en.wikipedia.org/wiki/Chemotherapy">chemotherapy</a> (<a title="Medication" href="http://en.wikipedia.org/wiki/Medication">drugs</a>) and <a title="Radiotherapy" href="http://en.wikipedia.org/wiki/Radiotherapy">radiotherapy</a> (<a title="Electromagnetic radiation" href="http://en.wikipedia.org/wiki/Electromagnetic_radiation">radiation</a>). However, immunotherapy is also often used in the immunosuppressed (such as <a title="HIV" href="http://en.wikipedia.org/wiki/HIV">HIV</a> patients) and people suffering from other immune deficiencies or autoimmune diseases.</p>

<dl><dd><em>See main article <a title="Diagnostic immunology" href="http://en.wikipedia.org/wiki/Diagnostic_immunology">Diagnostic immunology</a></em> </dd></dl><p>The specificity of the bond between antibody and antigen has made it an excellent tool in the detection of substances in a variety of diagnostic techniques. Antibodies specific for a desired <a title="Antigen" href="http://en.wikipedia.org/wiki/Antigen">antigen</a> can be conjugated with a radiolabel, fluorescent label, or color-forming enzyme and are used as a &quot;probe&quot; to detect it.</p>

<p>Study of the immune system in extant and <a title="Extinction" href="http://en.wikipedia.org/wiki/Extinction">extinct</a> species is capable of giving us a key understanding of the <a title="Evolution" href="http://en.wikipedia.org/wiki/Evolution">evolution</a> of species and the immune system.</p><p>A development of complexity of the immune system can be seen from simple phagocytotic protection of single celled organisms, to circulating antimicrobial peptides in insects to lymphoid organs in vertebrates. Of course, like much of evolutionary observation, these physical properties are often seen from the <a title="Anthropocentric" href="http://en.wikipedia.org/wiki/Anthropocentric">anthropocentric</a> aspect. It should be recognised, that every organism living today has an immune system absolutely capable of protecting it from most forms of harm; those organisms that did not adapt their immune systems to external threats are no longer around to be observed.</p><p><a title="Insect" href="http://en.wikipedia.org/wiki/Insect">Insects</a> and other <a title="Arthropod" href="http://en.wikipedia.org/wiki/Arthropod">arthropods</a>, while not possessing true adaptive immunity, show highly evolved systems of innate immunity, and are additionally protected from external injury (and exposure to pathogens) by their <a title="Chitin" href="http://en.wikipedia.org/wiki/Chitin">chitinous</a> shells.</p>

<li><a title="Immune system" href="http://en.wikipedia.org/wiki/Immune_system">immune system</a> </li> <li><a title="Autoimmunity" href="http://en.wikipedia.org/wiki/Autoimmunity">autoimmunity</a> </li> <li><a title="List of immunologists" href="http://en.wikipedia.org/wiki/List_of_immunologists">list of immunologists</a> </li> <li><a title="History of immunology" href="http://en.wikipedia.org/wiki/History_of_immunology">history of immunology</a> </li> <li><a title="Serology" href="http://en.wikipedia.org/wiki/Serology">serology</a> </li> <li><a title="Immunodeficiency" href="http://en.wikipedia.org/wiki/Immunodeficiency">Immunodeficiency</a> </li>