immunology Microbiology Pathology
Autoimmune diseases occur when the body’s immune system which is supposed to protect the body by mounting an immune response against foreign or external(non-self) antigens, mounts immune response against body’s own antigen. This results in extensive destruction of cells,tissue or organ against which immune response is directed.
There exists a protective mechanism(checkpoint) that prevents these dangerous T and B lymphocytes directed against self antigens. Auto-immunity develops when these check points are defective or these cells find a way to evade them.
Common autoimmune diseases and their pathogenesis are as follows:
Rheumatoid arthritis: IgM antibodies are produced that initiate chronic inflamation in target tissues like Joints and Kidneys.
Systemic Lupus Erythematosus: Antibodies are directed against DNA, histones, RBC, Platelets and other cellular components.
Multiple Sclerosis: Myelin, the covering of nerve sheaths, eseential for conduction is destroyed by auto immune attack
Type 1 Diabetes: Activated CD4 Lymphocytes infiltrate the islet of Langerhans. After infiltrating, these lymphocytes recognize peptides in islet cells, initiating inflamatory response and production of lytic enzymes destroying islet cells that secrete insulin.
Theories of Auto-Immunity:
These are the most widely accepted theories of auto-immunity that explain the process.
1.Certain tissues in the body remain sequestered from the immune machinery either due to lack of blood supply or protective layers around it. For example Lens,Myelin Basic Protein,Testes .
2.External pathogens may share antigenic components with host tissue or just mimic them. In this case, when an immune response is mount against the pathogen the antibodies also cross react with host tissues resulting in extensive damage. Rheumatic Fever is the classic example where Streptococcal antigens mimic heart muscle antigens exposed at areas of greater turbulence like valve leaflets and surrounding muscles.
3.Dysregulated expression of MHC class II molecules may inturn result in presentation of self antigens which in turn attracts immune response against these cells.
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