Category:Autoimmune Diseases

Introduction

Autoimmune disease is defined as a disease state characterised by a specific antibody or cell mediated response against the body's own tissues ('self' antigens).

The breakdown of tolerance to self antigens and the failure to regulate pathological immune responses are both responsible for autoimmune diseases. It has been shown in mice that thymectomy causes autoimmune disease, and plays a very important role in the recognition of 'self'.

Particular individuals may be more susceptible to autoimmune diseases due to:

Genetic factors

Many autoimmune diseases have a familiar component and the Human Leucocyte Antigen (HLA) haplotype is the predominant genetic factor, however having a particular HLA haplotype does not automatically result in the development of an autoimmune disease.

• HLA-DR3/DR4 (MHC Class II):

1. Diabetes Mellitus

2. Rheumatoid Arthritis

• HLA-B27 (MHC Class I):

1. Ankylosing spondylitis

• TNF alpha

• CTLA-4

Hormonal factors

Testosterone is protective against Systemic Lupus Erythematosus (SLE) and thus it is not seen in males.

Environmental factors

Infection

1. Molecular mimicry

• Some bacteria and viruses have antigens that resemble host-cell components and the body can then also initiate an immune response against itself.

2. Polyclonal activation

• T and B cells activation can occur as a result of an infection resulting in polyclonal activation. This can then cause autoreactive autoantibodies or mediate autoimmunity.

3. Inappropriate MHC expression

• Autoreactive T cells are activated because infection stimulates APC and upregulates MHC class II.

Diet

Stress

	Antibody mediated	Antibody mediated	Cell mediated
Type of hypersensitivity	Type II	Type III	Type IV
Pathogenesis	Antibody to cell surface or matrix antigens	Antibody to soluble self antigens	T cell and macrophage activation to self antigens
Examples of diseases	Immune mediated haemolytic anaemia (IMHA)	Systemic Lupus Erythematosus (SLE)	Rheumatoid arthritis
	Immune mediated thrombocytopaenia (IMTP)		Diabetes Mellitus type 1
	Myasthenia gravis		Hypothyroidism (lymphocytic throiditis)
	Pemphigus Vulgaris		Hypoadrenocorticism (Addisons disease)
	Bullous pemphigoid

Antibody mediated autoimmune diseases

1. Immune Mediated Haemolytic Anaemia (IMHA)

Pathogenesis: Type II hypersensitivity

• Autoantibody attack on the red blood cells causing extravascular and intravascular lysis which causes anaemia.
• The in saline autoagglutination test and the coombes test is used to diagnose the disorder.

2. Immune Mediated Thrombocytopaenia (IMTP)

Pathogenesis: Type II hypersensitivity

• There is an immunological attack on the platelets which reduces the ability for clotting.
• A reduced platelet count and detection of anti-platelet antibodies in the serum are used for diagnosis of this disorder.

3. Myasthenia gravis

Pathogenesis: Type II hypersensitivity

4. Bullous Pemphigoid

Pathogenesis: Type II hypersensitivity

5. Pemphigus Vulgaris

Pathogenesis: Type II hypersensitivity

6. Systemic Lupus Erythematosus (SLE)/Multi-systemic immune mediated disease

Pathogenesis: Type III hypersensitivity

Cell mediated autoimmune diseases

1. Rheumatoid arthritis

Pathogenesis:

• Type IV hypersensitivity: CD4 Th-1 cell mediated.
• Macrophages phagocytose self antigens and can present peptides on their MHC Class II molecules. The autoreactive TH-1 cells release IFN-gamma which activates macrophages to release prostaglandins, MMP enzymes and TNF-alpha (pro-inflammatory mediator), see diagram. These cytokines cause inflammation and tissue destruction.

2. Diabetes Mellitus type I

Pathogenesis:

• Type IV hypersensitivity
• The CTLs think that all the beta cells in the pancreas are infected by a virus, as it wrongly detects a self antigen presented by the MHC class I on the surface of the cell as foreign.
• Autoreactive CD8+ CTLs are inadvertently activated, destroying the beta cells, thus preventing the secretion of insulin and causing diabetes type 1 (see diagram).

3. Hypothyroidism (lymphocytic throiditis)

Pathogenesis: Type IV hypersensitivity

• T cell mediated

4. Hypoadrenocorticism (Addisons disease)

Pathogenesis: Type IV hypersensitivity