What is Immune system?
The immune system describes a set of the major defense mechanisms to protect the body against attack of harmful agents. It is a part of control and co-ordination. The resistance of the host towards a pathogenic microbe and its toxin is known as immunity. The type of immunity which is hereditary and is passed from one generation to the next is known as natural, innate or non-specific immunity. It includes nonspecific substances present in body fluids such as tissue lysozymes and antiviral interferons, nonspecific phagocytic cells and the integrity of the skin and the mucosae. On the other hand, the type of immunity which the host acquires during the course of its life time is called acquired or specific immunity. The basis for acquired immunity lies in the ability of the immune system to recognize cells or substances of their own bodies from those of other origins. Non-self materials include cells from other animals, viruses, toxins and toxoids, bacteria, and vaccines. Such materials entering the body are recognized as foreign substances or antigens, and they stimulate the production of humoral antibodies. These antibodies respond with specific foreign substance in order to remove or neutralize them.
Lymphocytes are the useful units of the immune system; they express their specific activity in two main ways. Firstly, certain types of lymphocyte produce antibodies in response to the detection of a particular antigen. Antibodies bind to antigens to prop up destruction of antigen by a variety of mechanisms; the defense mechanism mediated by antibody is called humoral immune response. Secondly, some lymphocytes are stimulated by antigens to produce a response in which circulating antibodies are not formed but in which lymphocytes and macrophages cooperate in the direct destruction of pathogenic organisms. This defence mechanism is called cellular immune response.
Lymphocytes are disseminated throughout the body either as isolated cells or diffuse aggregations particularly in the gastro-intestinal and respiratory tracts, or within the lymphoid organs.The thymus and bone marrow are the primary lymphoid organs in mammals.
During early fetal development blood cells are produced in the mesenchyme of the yolk sac. As the development of the fetus progresses the liver and spleen take over this role to maintain immune system. In later stages of fetal development, the bone marrow becomes the dominant marrow site of blood cell formation. The bone marrow gives rise to all of the lymphoid cells that migrate to the thymus for T-cell maturation as well as to the major population of conventional B-cells.
Thymus is a large lymphoid organ located in the anterior aspect of the thoracic cavity and the lower part of the neck. The major immune system activity of thymus takes place during childhood after which it gradually involutes in such a way that in human adults it becomes a hazy entity. The principal function of thymus is the production of immune-competent T-lymphocytes by proliferation and modification of basic lymphocytes produced by bone marrow. Thymectomy or congenital absence of a thymus results in no T-cells and a grossly deficient cell-mediated immune system.
Lymph nodes are small bean-shaped organs found at varying points along the lymphatic system, e.g. groin, armpit and mesentery. Each lymph node consists of a connective tissue framework and of numerous lymphocytes and other cells that fill the interstices of the network. Two major functions attributed to lymph nodes are:
(i) Non¬specific filtration of particulate matter and bacteria from lymph by the phagocytic activity of macrophages.
(ii) Storage and proliferation of B- and T-lymphocytes and antibody production.
Spleen is a large lymphoid organ situated in the left upper part of the abdomen. Like other lymphoid tissues, spleen is a centre where both B-and T- lymphocytes multiply, and it plays an important role in immune system responses. It also helps in the removal of debris and other particulate matter from circulating blood. The defective blood cells, particularly erythrocytes, are also removed from the circulation by the spleen.
Mucosa associated lymphoid tissues (MALT)
The main sites of entry for microbes into the body are through the epithelial surfaces containing mucosal epithelial cells. Therefore, over 50% of the lymphoid tissue in the human body is located within the lining of the major tracts, including respiratory, digestive and genito-urinary tracts. The lymphoid tissue associated with the throat and nasal passages (NALT, nasal-associated lymphoid tissue) are tonsils. They are similar to that of the lymph nodes and consist of B-cell follicles. Antigens and foreign particles are trapped within the deep crypts of their lympho-epithelium from where they are transported to the lymphoid follicles.
The gut-associated lymphoid tissue (GALT) is composed of lymphoid complexes which consist of specialized epithelium, antigen processing cells and intra-epithelial lymphocytes. These structures occur strategically at specific areas in the digestive tract. The primary role of GALT is to maintain the immune system and protect the body against microbes entering via the intestinal tract.