This shows a space filling model of mouse IgG2a from the strain BALB/c. The two heavy chains are shown in yellow and light blue whilst the two light chains are shown in green and dark blue. The carbohydrate (red and green) is just visible in the Fc.
So far only one crystal structure of a complete immunoglobulin has been solved. This was for the structure of a mouse monoclonal antibody of subclass IgG2a and was published by L.J.Harris, S.B.Larson, K.W.Hasel, J.Day, A.Greenwood, A.McPherson "The three-dimensional structure of an intact monoclonal antibody for canine lymphoma" Nature 360: 369 (1992) and a further refined structure published L.J.Harris, S.B.Larson, K.W.Hasel, A.McPherson, "Refined structure of an intact IgG2a monoclonal antibody", Biochemistry 36: 1581, (1997). The molecules identification reference in the protein databank is 1IGT.
One of the key things to note from the above space filling model is that the molecule has adopted an asymmetric conformation despite having two identical heavy and two identical light chains. This illustrates how the two Fab arms can take on different conformations with respect to each other and to the Fc region of the antibody. Flexibility in the IgG molecule is probably a very important feature for the functional activity of the molecule.
A rotating animation of the above structure is available in my animated molecules section