The dictionary defines ignorance thusly:
"The condition of being uneducated, unaware, or uninformed."
This is a common "condition" that can be found in the perspectives of everyone in some way or another. I guarantee that there are some things (most likely obscure and relatively unimportant) I know that you don't. In that one sense, you are more ignorant than me. On the other hand, the more refined our own respective fields of knowledge become, the more subjects there are that you know much more about that I do, making me more ignorant than you.
However, this is a more specific application of the term. If someone was to be called wholly ignorant (of everything), that would imply that they have no education whatsoever, are unaware of what's going on, and aren't informed about anything. In truth, I think there are very few people who fall into this category.
What I'm seeing a lot of though, especially in a climate of high political intensity, emotion, and name-calling, is this term being used as a general insult, applied in a general way over a dispute that is usually about subjects that are much more specific in nature.
It's like another way of calling someone "stupid" but in a slightly more grown-up and politically correct way. It's still intended to have the same kind of sting. Like I said, there are people who are (willingly or unwillingly) very ignorant, but I've come to believe that when the average person calls someone this, all they're really saying is "you don't know the same things I do." News flash. Does that really make a person ignorant in general? I don't think so.
In this case, a good response to this is "ignorant of what?" Get the person to give up their secrets - spill exactly what it is that they know that you don't. Most of the time, all this uncovers is that both people do indeed know things the other doesn't, but that this stems more from a difference of opinions and choice of sources rather than general stupidity.
Two people can both be completely informed, educated, and aware of the stories broadcast by their favorite news network, but if that network has a bias, or the news is inaccurate (which is the case to some degree with every network), then both people will call each other ignorant, simply for following different sources.
For example, global warming and its causes. I'm often annoyed by the scare tactics involved in spreading "awareness" about it, and also the apparent disconnect between this and the previous generation's certainty that we were on the verge of another ice age. But no matter. The point is, I will be the first one to admit that I'm ignorant about many of the sources of this hysteria. I don't know many scientists personally, nor do I spend large amounts of time reading and researching scientific data. I don't know who started this craze (besides Al Gore), who is currently studying it, how many scientists are involved, how many have differing opinions, and how many of these people are really qualified to be making such statements about earth's climate anyway.
Nor am I asking anyone to comment and give me all kinds of data :-P Because that's the point... what I've read about this subject comes from different sources than what many other people read - therefore we're all ignorant. If you're not a scientist doing all of this research yourself, then you're most likely getting any information from second or third-hand sources anyway. I'm also ignorant of many medical conditions. Really, the majority of us are, since very few people are actual physicians, and even physicians don't know everything. Even the "experts" (in any field) must ultimately apply their own interpretation to any data they may find.
There are also "experts" on every side of things. Ever watched a court trial, and seen the experts from both sides give completely opposite interpretations of an autopsy, or other kind of evidence? Who's ignorant? One or both?
Of course, I'm not saying there's an excuse for being willingly ignorant...for refusing to learn or at least hear other sides of things and take other perspectives into account. But I've found that calling someone ignorant just because they have a different opinion than me isn't very smart... it generally means I have three fingers pointing back at myself. The real difference is worldview - I don't believe humans have sole control of the universe, and therefore my views on global warming will automatically be different than someone who believes the opposite. But that doesn't mean I'm ignorant.