Which of the following statements about antibodies are correct? ( )

A、All antibodies are immunoglobulin.;
B、J chain can mediate the transport of SIg
A、;
C、Antibody can also be used as antigen.;
D、IgG is the main antibody that plays a role in anti-infection immunity.;
E、All antibodies are composed of two heavy chains and two light chains.