Anie,
Antibodies are usually found in the blood, they are part of the immune system and serve many purposes, protecting us from viruses and bacteria. B cells, which are a type of white blood cell produce antibodies. There are many different types of antibodies designed to interfere with different types of 'antigens' - which is what antibodies identify and attack. This is the necessary function that antibodies serve in our body. In the case of regularly transfused individuals, antibodies can develop against certain antigens that are present in the foreign blood. In severe cases the antibodies can cause severe transfusion reactions, and in other cases they can cause hemolysis (destruction of the transfused red blood cells) - these types of antibodies - specific for foreign blood are known as allo antibodies. It is not uncommon for thal majors to be identified with one of more allo antibody, careful screen of blood during cross matching can prevent any troubles from these antibodies. The production of antibodies can be avoided by doing phenotype matching (this needs to be done before beginning chronic transfusions), or genotype matching. This is so that exposure to possible antigens is reduced or eliminated.
In some cases, once a patient has formed multiple allo antibodies they may develop auto antibodies (which happened with my son 2 years ago). These antibodies are not specific for antigens from any particular donor - they attack all red blood cells. This can cause mild, moderate or severe hemolysis depending on the quantity of titers present in the blood.
Antibodies are also known as gamma globulin proteins and immunoglobulins. In my son's case he has been given rituximab, a drug that is specific for B cells, to temporarily stop the production of antibodies.
When we receive vaccinations, especially live vaccinations like measles or varicella (chicken pox) - our body is given an injection of a (very beaten up) virus that will duplicate a little in our body - our body identifies it and makes antibodies against it slowly. Once the body learns to make these antibodies, it remembers this antigen (that is why this part of our immune system is called anamnesic because of its tendency to remember) so when we are exposed to it in the future our body will quickly and rapidly destroy it before it can really make us sick.
I hope this helps explain the very important but sometimes pain in the butt antibodies,
Sharmin