Antigens help

What exactly are antigens? I thought they were foreign substance which causes an immune response but I read that autoimmune disease involves lymphocytes attacking their own antigens. If antigens are foreign, how can lymphocytes attack their "own" antigens?

@sam72016

The above answer gives you the principle correctly - let me add some meat to the skeleton if I may:

Certain parts of cells act as antigens rather than whole cells, and when the body's immune system fails to recognize its own antigens as "self", then it classifies them as "foreign" and antibodies are produced against them.

Let me give you some examples:
In a disease called systemic lupus erythematosus [SLE], the patient's immune system produces anti-nuclear, anti-DNA and anti-histone antibodies, which destroy the relevant substances - the disease is usually fatal due to renal failure or other severe organ damage. It occurs mostly in women of child-bearing age.

In myasthenia gravis, the body produces antibodies against its own nicotinic neuromuscular acetylcholine receptors (AChR), so that the contraction of muscle is interfered with - the patient gets exhausted later in the day as ACh gets depleted. If the ventilatory muscles are affected, respiratory failure can occur; dysphagia [due to interference with swallowing muscles in the pharynx] can also occur. Treatment is with a drug that inhibits the enzyme that breaks down ACh [pyridostignmine], thus increasing levels of ACh in the body, which helps a little, but the prognosis is poor (gravis is from gravity meaning seriousness).

In rheumatic fever, which follows an infection with type A beta-haemolytic streptococci (a type of bacterium), the body confuses the antigens on its own joints and heart cells with the antigens on the bacteria, so the antibodies attack these organs, causing arthritis and later cardiac valve disease.

I mention these diseases cos Qs sometimes come up in A level biology where some info is given about a disease and you have to combine this info with your own knowledge of biology to work out the answers to questions - if you already know the above, then you are one step ahead..

M (specialist biology tutor)

Alright thanks, but if it belongs to a body cell why is it called an antigen? I always thought of an antigen as a foreign molecule which binds to (friendly) antibodies so should I change my definition?

Alright thanks, but if it belongs to a body cell why is it called an antigen? I always thought of an antigen as a foreign molecule which binds to (friendly) antibodies so should I change my definition?

Strictly speaking, a specific antibody binds to its complementary antigen, not the other way round as you state. When you use the word "friendly" within parentheses, I suppose you mean friendly to ourselves - I would not use this word actually as it is confusing.

A good definition of an antigen is:

"A chemical substance, usually a protein, or structure of an organism on its surface that is seen by human recognition lymphocytes as foreign, so that EITHER specific antibodies are produced against this antigen, and these antibodies bind to the antigen, destroying it by precipitation or agglutination OR the cell exhibiting the antigen is phagocytosed by T lymphocytes, neutrophils or macrophages."

Sorry about the length of this definition - In fact, you could add opsonization to it!

YOUR FIRST Q:An antigen is just a label we humans have assigned to this molecule - if it so happens that the molecule happens to be chemically similar or even identical to one of our own chemical moieties (e.g. the joint and cardiac structures in the example of rheumatic fever in my above post), then our own antibodies might (attempt to) destroy it, leading to auto-immune disease.
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One last point to mention - we humans are not complete idiots [as some extreme animal lovers seem to think]- in the treatment of e.g. certain types of cancer, we have created drugs that include a specific antibody against these cancer cells, so that the patient's own normal cells are spared by the drug, which selectively destroys the cancer cells.

Hope this answers your follow-up Q.
M.

Absolute yes to both Qs, young man! One small addition: When the donor and recipient have similarities, then the probability of a transplanted organ e.g. kidney or heart being rejected is smaller; therefore, a live kidney donor is usually a relative of the patient (preferably a first-degree one i.e. parent, brother sister, child rather than e.g. uncle or grandparent); also, there are other ways of predicting the chances of rejection e.g. by comparing so-called HLA (human leucocyte antigens) antigens in the donor and the recipient, but this is well beyond A level, unless you are aiming for A* OR for the synoptic Q.

Best,
M.