The immune system is a system of biological structures (organs, tissues, cells and molecules) and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells [1]. Pathogens are found from substances that may be harmful, such as germs (virus, bacterium, prion, fungus), poisons and cancer cells [2]. These harmful substances have proteins called antigens on their surfaces. As soon as these antigens enter the body, the immune system recognizes them as foreign and attacks them. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism´s own healthy cells and tissues in order to function properly [2]. However, the immune system may also be related with the risks on autoimmune diseases, cancer [3,4] and graft rejection in organ or tissue transplants [5]. It occurs when the immune system mistakes the cells and tissues as a pathogen.
This paper describes role of immune system in preventing pathogenesis and treatment of disease. Specially, disorders in the immune system can result in disease, including autoimmune diseases, inflammatory diseases and cancer.
Cells of the innate immune system (non-specific) include natural killer (NK) cell lymphocytes, monocytes/macrophages, immature or dendritic cell precursors, neutrophils, basophils, eosinophils, tissue mast cells, and epithelial cells [2]. These cells recognize pathogen molecular motifs that are highly conserved among many microbes and use a diverse set of receptor molecules. Important components of recognition of microbes are: (1) recognition by germ line-encoded host molecules, (2) recognition of key microbe virulence factors, and (3) non-recognition of benign foreign molecules or microbes [2]. It is particularly important for macrophages and NK lymphocytes to may kill pathogens directly or may activate a series of events that both slow the infection or recruits other cells to kill the infected cells [1].
Adaptive immune system (specific) consists of T and B lymphocytes response that allows the development of true immunity to an infectious agent [1]. Adaptive immunity is found only vertebrate and is based on generation of antigen receptor T and B lymphocytes by germ-line gene rearrangement that occur during of each person [2]. Two major features define a specific immune response. First, specific immunity is only induced towards the agent as a specific pathogen, called the immunogen, which stimulated it and secondly, the specific immune response is more rapid following subsequent contacts with that same immunogen is known as immunological memory. This process is mimicked by the use of vaccines, where an attenuated or less virulent strain of the microorganism is administered to induce immunity without causing the disease [1].
Immunodeficiency diseases occur when the immune system is less active than normal, resulting in recurring and life-threatening infections. Immunodeficiency can either be the result of a genetic disease as severe combined immunodeficiency, or be produced by pharmaceuticals or an infection, such as the acquired immune deficiency syndrome (AIDS) that is caused by the retrovirus HIV [2]. In contrast, autoimmune diseases result from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto´s thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, psoriasis, and lupus erythematosus [2]. Immunology covers the study of all aspects of the immune system, having significant relevance to health and diseases [2]. Further investigation in this field is expected to play a serious role in promotion of health and treatment of diseases.
Immunodeficiencies occur when one or more of the components of the immune system are inactive. The ability of the immune system to respond to pathogens is diminished in both the young and the elderly, with immune responses beginning to decline at around 50 years of age due to immunosenescence [6,7]. In developed countries, obesity, alcoholism and drug use are common causes of poor immune function [6]. However, malnutrition is the most common cause of immunodeficiency in developing countries [7]. Diets lacking sufficient protein are associated with impaired cell-mediated immunity, complement activity, phagocyte function, IgA antibody concentrations, and cytokine production. Additionally, the loss of the thymus at an adult age through genetic mutation or surgical removal results in severe immunodeficiency and a high susceptibility to infection [8].
Immunodeficiencies can also be inherited or acquired [9]. Chronic granulomatous disease, where phagocytes have a reduced ability to destroy pathogens, is an example of an inherited, or congenital, immunodeficiency. AIDS and some types of cancer cause acquired immunodeficiency [10,11].
Autoimmunity is overactive immune responses that comprise the other end of immune dysfunction, particularly the autoimmune disorders. Here, the immune system fails to properly distinguish between self and non-self, and attacks part of the body. Under normal circumstances, many T cells and antibodies react with “self” peptides [12]. One of the functions of specialized cells (located in the thymus and bone) is to present young lymphocytes with self antigens produced throughout the body and to eliminate those cells that recognize self-antigens, preventing autoimmunity [13].
Hypersensitivity is an immune response that damages the body's own tissues. They are divided into four classes (Type I – IV) based on the mechanisms involved and the time course of the hypersensitive reaction. Type I hypersensitivity is an immediate or anaphylactic reaction, often associated with allergy, asthma and eczema. Symptoms can range from mild discomfort to death. Type I hypersensitivity is mediated by IgE, which triggers degranulation of mast cells and basophils when cross-linked by antigen [14]. Type II hypersensitivity occurs when antibodies bind to antigens on the patient's own cells, marking them for destruction. This is also called antibody-dependent (or cytotoxic) hypersensitivity, and is mediated by IgG and IgM antibodies [14]. Immune complexes (aggregations of antigens, complement proteins, and IgG and IgM antibodies) deposited in various tissues trigger Type III hypersensitivity reactions [14]. Type IV hypersensitivity (also known as cell-mediated or delayed type hypersensitivity) usually takes between two and three days to develop. Type IV reactions are involved in many autoimmune and infectious diseases, but may also involve contact dermatitis. These reactions are mediated by T-cells, monocytes, and macrophages [14].
In summary, the immune system may play a major role in immunity and treatment of disease that protects against pathogens, disease, and infectious agents by innate and adaptive immune response. However, disorders of the immune system can cause immunodeficiency diseases, inflammatory diseases (such as AIDS), allergic disorders, autoimmune diseases and cancers of the immune system.
References
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