According to the MedlinePlus Medical Encyclopedia (associated with NIH)
an antibody (also known as an immunoglobulin) is a protein produced by the body’s immune system when it detects harmful substances, called antigens.
Each antibody responds to and binds to a particular antigen. They are in the shape of a Y comprised of 4 polypeptides; 2 heavy chains and 2 light chains. Antibodies are produced by B-cells, a type of lymphocyte.
When healthy tissue is mistaken for something harmful and antibodies are produced an autoimmune disease can develop.
For more information:
- A Better Delivery System for Chemotherapy Drugs (scienceblog.com)
- Human monoclonal antibody development (slideshare.net)