Organs, cells, and proteins constitute the system and antigens are the molecules and organisms to which it responds. The structure of the protein of the immunoglobulin antibody molecule includes two identical but variable binding domains and a common region.
Genetic recombination provides essential diversity of antibodies and T-cell receptors.
Clonal selection provides essential biological economy. Vaccines are pseudo-antigens which prime the immune system. The immune system sometimes does not differentiate well between 'self' and 'non- self' resulting in autoimmune diseases. Allergies are an over-response of the immune system. The response is limited by the T-8 suppresser cells .