I'm sorry to hear of the situation.
I'm afraid that you have a bit of a problem. I understand what you are saying---trying to make out a case for self defense
----but self defense against some who spits is not tot to physically assault that person. The escalation of violence is what got you cited for the harassment. The law reads: § 2709. Harassment. (a) Offense defined.--A person commits the crime of harassment when, with intent to harass, annoy or alarm another, the person: (1) strikes, shoves, kicks or otherwise subjects the other person to physical contact, or attempts or threatens to do the same; (2) follows the other person in or about a public place or places; (3) engages in a course of conduct or repeatedly commits acts which serve no legitimate purpose; (4) communicates to or about such other person any lewd, lascivious, threatening or obscene words, language, drawings or caricatures; (5) communicates repeatedly in an anonymous manner; (6) communicates repeatedly at extremely inconvenient hours; or (7) communicates repeatedly in a manner other than specified in paragraphs (4), (5) and (6).
In Pennsylvania, Harassment is most often a summary offense. Those crimes are punishable by a fine up to $300 and/or up to 90 days in jail. Under the circumstances, it is highly unlikely that you will spend any time in jail, and you will simply receive a fine.
If you choose to plead not guilty and fight the charge, you will get the opportunity to prove that you did merely defend yourself and that the charge is not justified. Many of these summary charges can be won if you simply have a local defense attorney assist you in the defense.
Here is a good article on harassment charges in PA:
I hope that you found my answer informative, that you are accepting of my efforts and that you will rate my efforts based on the knowledge I have provided to you.
I wish you the best in 2012.