Immunology is a broad branch of biomedical science that covers study of all aspects of the immune system in all organisms. It deals with, among other things, the physiological functioning of the immune system in states of both health and disease; malfunctions of the immune system in immunological disorders (autoimmune diseases, hypersensitivities, immune deficiency, allograft rejection); the physical, chemical and physiological characteristics of the components of the immune system in vitro, in situ, and in vivo. Immunology has various applications in several disciplines of science, and as such is further divided.
Before even the concept of immunity 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 thymus, spleen, bone marrow, lymph vessels, lymph nodes and secondary lymphatic tissues such as tonsils and adenoids) and skin. The major organs, the thymus and spleen, are examined histologically only post-mortem during autopsy. However some lymph nodes, and secondary lymphatic tissues can be surgically excised for examination while patients are still alive.
Classical immunology ties in with the fields of epidemiology and medicine. It studies the relationship between the body systems, pathogens and immunity. The earliest written mention of immunity can be traced back to the plague of Athens in 430 BC. Thucydides noted that people who had recovered from a previous bout of the disease could nurse the sick without coming down with the illness a second time. Many other ancient societies have references to this phenomenom but it was not until the 19th and 20th centuries before the concept developed into scientic theory.
The study of 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 innate immune system , and acquired or adaptive immune system , the latter of which is further divided into humoral and cellular components.
The mainstay of classical immunology has been the interaction between antibodies and antigens. Without an understanding of the properties of these two biological entities, much of immunology would be non-existent.
In the 21st century though, immunology has broadened its horizons with much research being performed in the more specialised niches of immunology. This includes the immunological function of cells, organs and systems not normally associated with immune system, as well as the function of the immune system outside classical models of immunity.
Clinical immunology is the study of diseases caused by the immune system and diseases of the immune system from a medical perspective.
Many diseases caused by the immune system fall into two broad categories: immunodeficiency, in which parts of the immune system fail to provide an adequate response (examples include chronic granulomatous disease), and autoimmunity, in which the immune system attacks its own antigens (examples include systemic lupus erythematosus, rheumatoid arthritis, Hashimoto's disease and myasthenia gravis). Other immune system disorders include different hypersensitivities, in which the system responds inappropriately to harmless compounds (asthma and allergies) or responds too intensively.
The most well-known disease that affects the immune system itself is AIDS, caused by the HIV virus. AIDS is an immunodeficiency characterized by the lack of CD4+ ("helper") T cells and macrophages, which are destroyed by the HIV virus.
See main article Immunotherapy
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 cancers together with chemotherapy (drugs) and radiotherapy (radiation). However, immunotherapy is also often used in the immunosuppressed (such as HIV patients) and people suffering from other immune deficiencies or autoimmune diseases.
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 antigen can be conjugated with a radiolabel, fluorescent label, or color-forming enzyme and are used as a "probe" to detect it.
Well known applications of this include immunoblotting , ELISA and immunohistochemical staining of microscope slides. The speed, accuracy and simplicity of such tests has led to the development of rapid techniques for the diagnosis of disease, microbes and even illegal drugs in vivo (of course tests conducted in closed environment have a higher degree of accuracy). Such testing is also used to distinguish compatible blood types.
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, this is often seen from the anthropocentric aspect, but it must be recognised, that every organism's immune system is absolutely capable of protecting it from most forms of harm.
Insects and other arthropods, 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 chitinous shells.