AUTOIMMUNE DISEASES

AUTOIMMUNE DISEASES

Autoimmune disease is defined as a condition in which the immune system mistakenly attacks body’s own cells and tissues. Normally, an antigen induces the immune response in the body. The condition in which the immune system fails to give response to an antigen is called tolerance. This is true with respect to body’s own antigens that are called self antigens or autoantigens. Normally, body has the tolerance against self antigen. However, in some occasions, the tolerance fails or becomes incomplete against self antigen. This state is called autoimmunity and it leads to the activation of T lymphocytes or production of autoantibodies from B lymphocytes. The T lymphocytes (cytotoxic T cells) or autoantibodies attack the body’s normal cells whose surface contains the self antigen or autoantigen. Thus, the autoimmune disease is produced when body’s normal tolerance decreases and the immune system fails to recognize the body’s own tissues as ‘self’.

Autoimmune diseases are of two types:

1. Organ specific diseases which affect only one organ

2. Organ nonspecific or multisystemic diseases, which affect many organs or systems.

HUMAN LEUKOCYTE ANTIGEN SYSTEM AND AUTOIMMUNE DISEASES

Human leukocyte antigen (HLA) is a group of genes on human chromosome 6. These genes encode the

proteins which function in the cells to transport the antigens from within the cell towards the cell surface.

HLA is the product of major histocompatilility complex. HLA system monitors the immune system in the

body (see above). The HLA molecules are recognized by the T and B lymphocytes and hence the name called antigens. HLA is distributed in almost all the tissues of the body. Antibodies are directed against the tissues possessing the HLA, leading to autoimmune diseases. Most of the autoimmune diseases are HLA linked.

COMMON AUTOIMMUNE DISEASES

Common autoimmune diseases are:

1. Insulin-dependent diabetes mellitus

2. Myasthenia gravis

3. Hashimoto thyroiditis

4. Graves disease

5. Rheumatoid arthritis.

1. Insulin-dependent Diabetes Mellitus

Insulin-dependent diabetes mellitus (IDDM) is very common in childhood and it is due to HLA-linked

autoimmunity.

Common causes for IDDM

i. Development of islet cell autoantibody against β-cells in the islets of Langerhans in pancreas.

ii. Development of antibody against insulin and glutamic acid decarboxylase.

iii. Activation of T cells against islets.

2. Myasthenia Gravis

This neuromuscular disease occurs due to the development of autoantibodies against the receptors

acetylcholine in neuromuscular junction

3. Hashimoto Thyroiditis

Hashimoto thyroiditis is common in the late middle-aged women. The autoantibodies impair the activity of thyroid follicles leading to hypothyroidism.

4. Graves Disease

In some cases, the autoantibodies activate thyroid-stimulating hormone (TSH) receptors leading to hyperthy roidism.

5. Rheumatoid Arthritis

Rheumatiod arthritis is the disease due to chronic inflammation of synovial lining of joints (synovitis). The synovium becomes thick, leading to the development of swelling around joint and tendons. The characteristic symp toms are pain and stiffness of joints. The chronic inflammation occurs due to the continuous production of autoantibodies called rheumatoid arthritis factors (RA factors).

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