Immune system disorders
The immune system is a multicomponent interactive system. It effectively protects the host from various infections. But an improper functioning of the immune system can cause discomfort, disease or even death. The improper functions fall into the following major groups :
(1) hypersensitivity or allergy,
(2) auto-immune diseases, and
Allergies result from an inappropriate and excessive immune response to common antigens. Substances that cause allergies are called allergens ; they include dust, moulds, pollen, certain foods, and some medicines (such as penicillin). Allergy involves mainly IgE antibodies and histamine. Mast cells secrete the histamine. A common manifestation of allergy is asthma. Some-times an allergen may cause a sudden, violent and fatal reaction in a sensititive individual ; this is called anaphylaxis.
Autoimmune diseases result when the immune system attacks and destroys 'self' cells and molecules. This condition can cause chronic and serious diseases. Examples of autoimmune diseases are insulin-dependent diabetes, multiple sclerosis, rheumatoid arthritis, etc. Multiple sclerosis is caused by antibodies that attack the myelin sheath of nerve cells.
Immunodeficiency Diseases result from a defect in one or more components of the innate or adaptive immunity. Affected individuals are susceptible to diseases that normally would not bother most people. Immunodeficiency may result from gene mutations, infections, malnutrition or accidents.
a. Severe combined immunodeficiency (SCID) results from one of many genetic defects; one such genetic defect leads to adenosine deaminase deficiency. SCID is characterized by a very low number of circulating thymocytes. Affected individuals usually die at an early age.
b. AIDS (Acquired Immuno Deficiency Syndrome) is another example for immunodeficiency disease. It is caused by a retrovirus, known as human immunodeficiency virus (HIV). Retroviruses have RNA genomes that are replicated via DNA copies. HIV, selectively infects and kills T-helper cells. The depletion of T-helper cells weakens the acquired immune response and may even abolish it completely. The viral RNA genome is converted into DNA copy by the viral enzyme reverse transcriptase. The DNA copy of HIV becomes inserted into the human chromosome and replicates with the cell DNA. It may be transcribed to produce RNA copies of the viral genome. The RNA copies are packaged and liberated as virus particles. The infected cell is lysed in this process, and the released virus particles infect new T-helper cells.