Clonal selection theory explains the function of lymphocytes in the immune system in response to particular antigens invading the body. It attempted to explain the formation of a diversity of antibodies during initiation of the immune response. The theory has become a widely accepted model for how the immune system responds to infection and how certain types of B and T lymphocytes are selected for destruction of specific antigens.
The theory states that in a pre-existing group of lymphocytes (B cells), a specific antigen only activates (i.e. selection) its counter-specific cell so that that particular cell is induced to multiply (make clones) for antibody production. In short the theory is an explanation of the mechanism for the generation of diversity of antibody specificity.