Immunological tolerance is the ability of an individual's immune system to ignore "self" while reacting to "non-self" organisms and substances.

The system is extremely complex, and needs to somehow set a balance between being able to accurately sense and eradicate "non-self" entities, whilst leaving the "self" cells etc. untouched.

Malfunctions of the system, in which it over-sensitively targets "self" structures, are believed to be of profound importance with autoimmune diseases such as Rheumatoid Arthritisplugin-autotooltip__plain plugin-autotooltip_bigRheumatoid Arthritis

Rheumatoid Arthritis (RA) is a chronic severe inflammatory disease believed to affect around 1% of the global population. It primarily affects joints,which typically become warm, swollen and painful. Pain and stiffness often worsen following rest. Most commonly, the wrists and hands are involved, with the same joints typically involved on both sides of the body.andLupusplugin-autotooltip__plain plugin-autotooltip_bigLupus

"Systemic lupus erythematosus, also called SLE or lupus, is a disease of the immune system that causes the immune system to attack tissues in various parts of the body. The cause of lupus is unknown. Finding the cause is the object of major research efforts. etc.

The mechanisms by which immunological tolerance is maintained in balance (and how it sometimes goes out of balance) are unclear. There are currently five main theories which attempt an explanation . They are the :

• Clonal deletion theory
• Clonal anergy theory
• Idiotype network theory
• Clonal ignorance theory
• Suppressor population theory

For details, see: Wikipedia

Also see Immune tolerance in pregnancyplugin-autotooltip__plain plugin-autotooltip_bigImmune tolerance in pregnancy

Why doesn't an expectant mother's immune system damage her growing baby - since it has 'foreign' DNA?

This fundamental phenomenon has yet to be fully explained.

"During pregnancy, the maternal immune system is exposed to a major challenge. The fetus expresses paternal alloantigens, yet it is not rejected.

Importance Rating