Antibodies have heretofore been often utilized in the detection of target molecules in various research and development activities, and are also of great industrial importance as detection reagents or diagnostic drugs.  The antibodies have also received attention as drugs for the treatment of diseases because of their specificity for target molecules. 
Chemical modifications for the functionalization of antibodies have been practiced, including modification with an enzyme such as alkaline phosphatase (AP) or peroxidase (HRP) (Non Patent Literatures 1 and 2), iodation or addition of a chelating compound for radioisotopes (Non Patent Literature 3), and modification with a low-molecular compound such as biotin (Non Patent Literature 4).  These modifications are typically performed via a lysine amino group, a cysteine thiol group, and an activated carboxyl group, etc.  These modifications are specific for the functional groups, but are not site-specific.  Therefore, the problems of these approaches are, for example, reduction in the activity of antibodies due to the modification or the like of the antigen-binding sites of the antibodies, and difficult control of the number of compounds to be bound.  For antibody-drug conjugates (ADCs) of antibody drugs (Non Patent Literatures 5 and 6), which have emerged in recent years, anticancer agents are bound to antibodies in a site-nonspecific manner.  Unfortunately, this weakens the activity of the antibodies themselves, or complicates subsequent steps of formulation due to difficult control of the number of anticancer agents to be bound, for example. 
In order to overcome these problems, antibody modification has been practiced using antibodies having a particular site-specifically introduced functional group.  For example, modification at a particular site is achieved by introducing a non-natural amino acid (Non Patent Literatures 7 to 9) or free cysteine (Non Patent Literatures 10 and 11) to the particular site by genetic manipulation.  Also, it has been reported that modification targeting particular natural or artificially introduced glutamine in antibodies is performed by using transglutaminase (TG) (Non Patent Literatures 12 and 13).  It is however known that the reaction yields are largely influenced by the structure of a compound to be introduced or the spatial environment of the targeted glutamine residue.  Furthermore, modification techniques targeting a sugar chain on antibody Fc have also been utilized (Non Patent Literatures 14 and 15).  These methods are site-specific, but require the oxidative modification of the sugar chain, and thus there is a problem that the reaction steps are complicated.  Although site-specific antibody modification techniques are under development as mentioned above, these methods often require engineering antibodies themselves and are not always advantageous in light of reduction in the functions of the antibodies and high development cost in association with the engineering.