Feedback

Understanding Auto-Immunity: When the Body Turns Against Itself

uto-immunity is a complex phenomenon where the body’s immune system mistakenly attacks its own tissues. This article delves into the mechanisms behind auto-immune diseases, the role of self-tolerance, and the classification of such conditions.

What is Auto-Immunity?

The Body's Defense System Gone Wrong

Under normal circumstances, the immune system identifies and attacks foreign invaders like viruses and bacteria. However, in auto-immunity, this system fails to distinguish between "self" and "non-self," leading to the production of antibodies and sensitized T-lymphocytes that target the body's own tissues.

• Self-Tolerance: During embryonic development, the immune system learns to recognize the body’s own tissues as "self." This recognition is crucial for preventing the immune system from attacking its own cells.
• Breakdown of Self-Tolerance: If self-tolerance mechanisms fail, auto-antibodies and auto-reactive T-cells can be produced, leading to autoimmune diseases.

Mechanisms Behind Auto-Immune Diseases

1. Sequestered Antigen

Certain body tissues, such as the lens of the eye and the central nervous system, are normally hidden from the immune system. If these sequestered antigens are exposed due to injury or disease, they can trigger an immune response.

2. Altered Antigen

Tissue antigens can be altered by injury, disease, or mutation, making them unrecognizable as "self." This can lead to the immune system attacking these altered antigens, as seen in conditions like rheumatoid arthritis.

3. Heterophile or Cross-Reacting Foreign Antigens

Some infectious agents have antigens that resemble the body’s own tissues. For example, antibodies produced against Streptococcus pyogenes may also attack the heart’s muscle tissue, leading to rheumatic heart disease.

4. Emergence of Forbidden Clones

When the immune system's regulatory mechanisms fail, genetically abnormal clones of immune cells may arise, capable of attacking the body's own tissues.

5. T and B Cell Defects

Defects in T or B cells, or inappropriate expression of certain proteins on antigen-presenting cells, can lead to a loss of self-tolerance and the development of autoimmune diseases.

Classification of Auto-Immune Diseases

Systemic vs. Organ-Specific Auto-Immune Diseases

Autoimmune diseases can be classified into two broad categories:

Systemic Autoimmune Diseases: These diseases involve multiple organs and tissues. Examples include systemic lupus erythematosus (SLE) and rheumatoid arthritis.

Organ-Specific Autoimmune Diseases: These conditions target specific organs. Examples include type 1 diabetes mellitus (pancreas), Graves' disease (thyroid), and autoimmune hemolytic anemia (red blood cells).

Pathogenesis of Auto-Immune Diseases

How Autoimmune Diseases Cause Damage

Autoimmune tissue injury can be mediated by:

• Humoral Antibodies: These antibodies circulate in the blood and can directly target and destroy cells, as seen in autoimmune hemolytic anemia.
• T-Cell Mediated Reactions: T-cells can also cause tissue damage, although this is less well understood compared to antibody-mediated damage.

Both Type II cytotoxic reactions (where antibodies target cells) and Type III immune complex diseases (where immune complexes are deposited in tissues) are involved in the pathogenesis of autoimmune diseases.