Alpha heavy chain protein molecular structure of IPSID (B) compared with normal IgA molecule (A). The dotted areas represent deleted variable and first constant regions of amino terminus of the heavy chain (blue) as well as missing entire light chain (orange). The normal structure of the immunoglobulin molecule resumes at the beginning of the hinge region (green). The carboxy terminal (COO^-) portion of the polypeptide is intact. Joining chains, present in a majority of the IPSID α heavy chain proteins, are not shown in the diagram.¹⁶