AUTOIMMUNE DISEASES, AUTOIMMUNITY AND AUTOANTIBODIES: DEFINITION, TYPES AND CLINICAL IMPORTANCE

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Autoimmune diseases are diseases that arise when the immune system of a host mounts an immunological attack against self-molecules. They are diseases which are caused by an immune attack against an individual’s own body tissues or cells. In autoimmune disease conditions or autoimmunity, the host’s immune system produces immunoglobulin molecules or antibodies that recognizes and attack the normal body tissues of the individual, instead of attacking invading antigens. Such antibodies produced against the host cells or tissues are generally known as autoantibodies.  Autoimmune disease is what happens in an individual when the immune system malfunctions (i.e., when the immune system of a host fails in its prime duty of distinguishing between self-molecules and non-self molecules).

The ability of the host’s immune system to unmistakably discriminate between self-molecules and non-self molecules (including pathogens and other foreign bodies) is one of the most vital attribute of the immune system of man and other related vertebrates. This characteristic of the immune system is important for the individual to mount specific immunological responses against an invading pathogen or antigen which it eliminates. Self molecules must be tolerated and differentiated from invading microbes and other foreign bodies by the hosts’ immune system so that autoimmune disease does not develop. Otherwise, when components of the immune system begin to attack host self molecules, autoantibodies will be generated, and this will lead to the development of autoimmune diseases or autoimmunity in the affected individual.

Autoantibodies as aforementioned are generally described as antibodies produced by a host immune system, and which recognizes and binds to the antigenic determinant or epitope of the individual’s normal body tissues (i.e., self molecules). In an autoimmune condition, self-reactive B cells and T cells are produced and these immune system cells produce effector molecules such as antibodies that have affinity for the normal tissues of the body. Autoimmune diseases are typical consequences of immune intolerance i.e., a situation in which the hosts immune system fails to distinguish between normal host self-molecules and non-self molecules or antigens. The terms autoimmunity and autoimmune diseases are sometimes used synonymously even though there may be some variation between the two terminologies.

While autoimmunity, a benign health condition describes the presence of autoantibodies in the serum of an individual; autoimmune disease, a malignant and often fatal disease condition is the self-inflicted immune system disease caused as a result of the activation of autoantibodies against the normal tissues or self molecules of the body. Autoimmune diseases could be: systemic autoimmune disease or organ-specific autoimmune disease ‘in nature’ depending on the part of the body that is affected.

There are several autoimmune immune diseases or disorders which affect humans and these include: Rheumatoid Arthritis, Hashimoto’s Thyroiditis, Graves disease, Systemic Lupus Erythematosus (SLE), Addison’s disease, Goodpasture’s syndrome, Pernicious anaemia, Myocardial infarction, Multiple sclerosis, autoimmune haemolytic anaemia, Idiopathic Thrombocytopenia Purpura and Ankylosing spondylitis (Table 1).

Table 1. List of some autoimmune diseases or disorders in humans

*SLE- Systemic Lupus Erythematosus, *AHA-Autoimmune haemolytic anaemia, *ITP-Idiopathic Thrombocytopenia Purpura

Autoimmune diseases affect the tissues, organs and cells of the body; and they mainly arise following a dysfunction or anomaly in the host immune system – thus causing the body to attack its own self. Though the aetiology of autoimmune diseases may still be obscure, several factors have been implicated to be responsible for their development. Autoimmune diseases are mainly treated clinically using some selective and less-toxic immunomodulatory agents that help the host’s immune system to selectively control and stop the attack on self molecules. Some of the reasons responsible for failure of the immune system to fine-tune its responses to recognize and attack only microbial pathogens or foreign bodies and antigens may include:

  • Immunological mimicry – in which some pathogens or antigens bear similar epitopes with that of the host’s tissues, cells or organs. This causes the immune system to respond to self-molecules because the antigens cross-react with the normal tissues of the body.
  • Mutation in immunocompetent B cells or T cells – This causes immune system cells to direct their immunological responses to the host’s cells, tissues or organs.      
  • Failure of the immune system to destroy or eliminate immune system cells that express immunological responses to host cell during clonal selection and expansion in the bone marrow or thymus as the case may be. Self-reactive antibodies and other immune system cells are usually destroyed by apoptosis or clonal anergy during B cell and T cell development and maturation. This occurs during clonal selection and expansion in order to ensure that the effector cells of the B and T lymphocytes produced during maturation only have affinity for foreign molecules or pathogens and not the host’s own tissues, organs or cells. Failure of the immune system to maintain this function may result in autoimmune diseases in the body. Apoptosis is defined as programmed cell death.  
  • Failure of the immune system to regulate its own response. This phenomenon is known as the loss of immunoregulatory functions of the immune system especially those provided by the T suppressor (TS) cells – whose function is to suppress the aggravated and unwarranted responses of the immune system in a host.     
  • Other factors that may give rise to the development of autoimmune diseases may be genetic factors, nutritional factors, chemical factors, environmental factors, physiological factors and stress factors.

Autoimmunity or autoimmune diseases is what happen in the body when the host’s immune system fails to respond in its normal physiological and immunological pattern especially in the discrimination of self-molecules (normal body tissues) and non-self molecules (i.e., antigens or pathogens). The immune system of the host is said ‘to go berserk’ in autoimmune disease conditions. When the host’s body or immune system loses its tolerance to self-molecules, immune reactions to the host’s own tissues or antigens is bound to occur; and this generally leads to autoimmunity or autoimmune diseases (Table 1). Autoimmunity precedes autoimmune diseases as aforementioned because when the immune system of the host malfunctions it will not be able to distinguish between self-molecules and non-self molecules; and this condition (i.e., autoimmunity)which permits an immune attack against the host’s tissues is what causes several chronic and acute debilitating diseases known as autoimmune diseases in the affected individual.

Autoantibodies, the precursors of autoimmunity are always present in the serum and/or clinical samples of individuals suffering from autoimmune diseases; and it is these autoantibodies found in the serum and/or tissues of the individual that causes autoimmune disorders or diseases. While systemic autoimmune diseases are widespread in the body and can affect a wide variety of cells, tissues and organs in the body; organ-specific autoimmune diseases only affect a particular organ of the body. The classification of autoimmune diseases as SYSTEMIC- and ORGAN-SPECIFIC AUTOIMMUNE DISEASES is usually based on the location of the disease in the body i.e., the particular site of the body where the disease is occurring. The list of autoimmune diseases enumerated in Table 1 as seen above is not exhaustive – since there are many types of autoimmune disorders or diseases that occur in humans especially when there immune system malfunctions.

Further reading

William E.P (2003). Fundamental Immunology. 5th edition. Lippincott Williams and Wilkins Publishers, USA.

Stevens, Christine Dorresteyn (2010). Clinical immunology and serology. Third edition. F.A. Davis Company, Philadelphia.

Silverstein A.M (1999). The history of immunology. In Paul, WE (ed): Fundamental Immunology, 4th edition. Lippincott Williams and Wilkins, Philadelphia, USA.

Paul W.E (2014). Fundamental Immunology. Seventh edition. Lippincott Williams and Wilkins, USA.

Male D, Brostoff J, Roth D.B and Roitt I (2014). Immunology. Eight edition. Elsevier Saunders, USA.

Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.

Berzofsky J.A and Berkower J.J (1999). Immunogenicity and antigen structure. In Fundamental Immunology, 4th edition., W.E. Paul, ed., Lippincott-Raven, Philadelphia. 

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