The immune system continuously protects us from attacks by various pathogens. It is distributed throughout the entire organism and consists predominantly of solitary cells. Nevertheless, it is so well coordinated that the immune system can be considered a single fascinating organ. In a simplified scheme, the elements of the immune system can be divided into four categories. Cellular can be distinguished from humoral and nonspecific, innate from specific components. The specific cellular immune system consists mainly of T and B lymphocytes. The B lymphocytes, in turn, develop into plasma cells in an appropriate immune response, which produce antibodies. Antibodies (Ab) form the specific, humoral part of the immune system. An important prerequisite for Ab-mediated immunity is the specific, non-covalent binding between Ab and antigens (AGs).