Agglutination not only requires complementary antigens and antibodies (creating an immune complex), it also hinges upon the degree of attraction between the antigens and antibodies and the strength of the bonds created in the formed immune complex.
Affinity refers to the attraction that a specific antibody possesses to its corresponding antigen. A high degree of specificity equates to bonding of that antibody to a specific antigenic epitope. Naturally occurring ABO antibodies possess a high degree of specificity to their antigenic counterparts.
Avidity refers to the strength that an antigen-antibody immune complex possesses at that epitope. Naturally occurring ABO antibodies have a high degree of avidity, resulting in a powerful paratope-epitope bond.