By Vito Quaranta , Maurizio Zanetti , and Ralph

Ant i id io typ ic an t ibodies have been used in various systems to s tudy bo th the spec t rum of the i m m u n e response (1) and the na tu re of cel lular receptors to ant igens (2). Because of the evidence tha t id io typic de t e rminan t s can be associated wi th the an t igen-combin ing site (3), i d io typy of a n t i b o d y m a y be used as a mi r ro r image for def ining ant igenic de t e rminan t s of molecules otherwise difficult to e lucidate . Moreover, the demons t ra t ion of id io typic markers on lymphocy te receptors (2) suggests that an t i id io typ ic (anti-receptor) an t ibodies can be used for funct ional studies of immune recognit ion. Ia molecules, H L A D R in man , are an extensively po lymorph ic fami ly of cellsurface ant igens of key impor t ance in regu la t ing an t igen responsiveness a n d i m m u n e cell coopera t ion (4). F rom studies in the mouse system (5), it appears tha t single Ia ant igenic de te rminan t s can affect recogni t ion events and i m m u n e per formance . In man , the defini t ion of single Ia de t e rminan t s cannot be achieved th rough genet ic r ecombina t ion or isolat ion of m u t a n t strains, as in the mouse, thus posing a ma jo r p rob lem for the funct ional s tudy of Ia antigens. W e describe here an ini t ial invest igat ion of the id iotypes of mur ine monoclona l an t ibodies (mAb) to h u m a n Ia ant igens, in an effort to define Ia de t e rminan t s and their cel lular in terac t ion pa thways .