Can I ignore a Summons in a Copyright Infringement Case?
You should NEVER ignore a Summons received in any legal proceeding, including a copyright infringement case. If you are served with a Summons, you have between 21 and 30 days to file paperwork (a "responsive pleading") in court to protect you from a default judgment. In a federal civil court case, the time is 21 days to file the proper paperwork in court. In State court, the time to file the proper paperwork in court will vary depending on the jurisdiction.
A Subpoena, on the other hand, is different from a Summons. Oftentimes, people receive a copy of a subpoena issued to their Internet company (ISP) along with a letter from their Internet company alerting them to the Subpoena. You should consult with an attorney if you received an actual Subpoena issued to you. And, you should also consult with an attorney if you receive a copy of a subpoena issued to your Internet company.
Also, beware of any letter asking you to sign and return a "Waiver of Service of Summons". If you sign the document and send it to the lawyer who sent it to you, you are voluntarily subjecting your self to the jurisdiction of the court just as if you were physically served with the Summons. Instead, seek legal advice immediately.
Again, you should never ignore a Summons, or even a Subpoena. Ignoring a Summons can result in an unfair default judgment being entered against you.