Definition of Antigen and Its Meaning in Lupus

Learn Which Two Antigens Are Key for Diagnosing Lupus

Systemic lupus erythematosus. Lupus is an autoimmune disease. Credit: Biophoto Associates / Getty Images

An antigen is substance that enters the body and stimulates the production of an antibody to fight what the immune system perceives as an invader.

Antigens can be toxins, bacteria, foreign blood cells and the cells of transplanted organs. Other antigens, called autoantigens, or self-antigens, are usually normal proteins in the body that are targeted by the immune system of lupus patients as foreign bodies.

In other words, normal tolerance of these autoantigens has been lost in lupus patients, mainly to genetic and environmental factors.

More on Antigens in Lupus

Lupus is a systemic, or whole-body, autoimmune disease that affects a number of different organs like the kidneys, joints, skin, nervous system, blood cells, and blood vessels. When one of these organs is being attacked by a person's immune system, signs and symptoms related to that organ are manifested. For instance, if the immune system of a person with lupus is producing antibodies that are attacking the kidneys, symptoms like protein in the urine (which can produce foamy urine), high blood pressure, and/or a rise in the blood creatinine level often occur. 

The launch of an organ attack starts with the immune system thinking that a self-antigen (like a normal protein in the body) is something foreign and bad. The recognition of the antigen by the body requires a combination of events, like a genetic predisposition and one or more environmental triggers, like an infection.

In other words, it takes a number of coincidental, unfortunate events -- a perfect storm so to speak.

Self-Antigens in Lupus 

In lupus, there are a number of potential self-antigens (also called autoantigens) that can stimulate an autoantibody response. 

These include:

  • intranuclear antigens 
  • double-stranded DNA (dsDNA)
  • Smith antigen (Sm)
  • U1 RNP antigen
  • Ro and La antigen
  • antiribosomal P proteins

Antibodies against intranculear antigens, called anti-nuclear antibodies (ANA) are highly sensitive for lupus, but not specific. This means that the majority of people with lupus have a positive ANA, but testing positive for ANA does not mean you have lupus. A positive ANA can signal another rheumatological condition, or mean nothing (some people just test positive.)

On the other hand, antibodies against the self-antigen dsDNA (called anti-disDNA) antibodies and those against the Sm antigen are very useful for diagnosing lupus, and are part of the American College of Rheumatology classification criteria for lupus. 

Anti-dsDNA antibodies tend to fluctuate with disease activity (meaning they are present when a person is having a lupus flare and are absent during a lupus remission) and are also linked to active kidney disease in patients with lupus. While anti-Sm antibodies are also very helpful for ruling in the diagnosis of lupus, they tend to remain positive (or be present) even when a person is in remission.

 

The Bottom Line

Antigens stimulate an immune response in the body -- the production of antibodies. Self-antigens like double-stranded DNA and the Sm antigen can produce antibodies that are highly specific for diagnosing lupus. 

Sources:

Fattal, I., et al. (2010). An antibody profile of systemic lupus erythematosus detected by antigen microarray. Immunology, July;130(3):337-43.

Riemekasten, G., & Hahn, BH. (2005). Key autoantigens in SLE. 6th European Lupus Meeting, Rheumatology, 44:975-82. 

YS Lee, A., & BH Ang, E. (2014). A clinical overview of autoantibodies in general practice rheumatology. British Journal of General Practice, Sep;64(626):e599-e601. 

DISCLAIMER: The information in this site is for educational purposes only. It should not be used as a substitute for personal care by a licensed physician. Please see your doctor for diagnosis and treatment of any concerning symptoms or medical condition.

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