The exons of these proteins attach via non-homologous end joining to perform class-switching. The "12/23" rule achieves efficient V(D)J recombination of these proteins. The two heavy and two light (*) chains that comprise them vary among the five isotypes of them that exist for mammals. Their epitopes are uniquely recognized by the molecules they form an induced fit with. 
The exons of these proteins attach via non-homologous end joining to perform class-switching. The "12/23" rule achieves efficient V(D)J recombination of these proteins. The two heavy and two light (*) chains that comprise them vary among the five isotypes of them that exist for mammals. Their epitopes are uniquely recognized by the molecules they form an induced fit with. These are secreted by B cells. For 10 points, identify these proteins used by the immune system against foreign antigens.
The exons of these proteins attach via non-homologous end joining to perform class-switching.
{antibodies} [or {antibody}; or {immunoglobulins}; or {Ig}]