Escape special characters in SED

Need help in escaping special characters in sed command.

Here is the the string which i am trying to find a replace with
From :-

REQUEST_TYPE=PIXEL&MSG_ID={//MESSAGE_ID}

To :-

REQUEST_TYPE=PIXEL&MSG_ID= X_EDELIVERY_MESSAGE_ID & BATCH_ID= X_EDELIVERY_BATCH_ID

Here is the sed command i am using.

sed "s/REQUEST_TYPE=PIXEL\&\amp\;\MSG_ID\=\\{\\/\\/\MESSAGE_ID\}\/REQUEST_TYPE=PIXEL\&\amp;MSG_ID= X_EDELIVERY_MESSAGE_ID \&\amp; BATCH_ID= X_EDELIVERY_BATCH_ID/g" $filename

Please advise where i am going wrong. Thanks a bunch.

sed 's#\(REQUEST_TYPE=PIXEL&MSG_ID=\){//MESSAGE_ID}#\1 X_EDELIVERY_MESSAGE_ID \& BATCH_ID= X_EDELIVERY_BATCH_ID#g' "$filename"

Regards,
Alister

Thanks a bunch..it worked..
Don't mind can you please explain the command..