How does one best respond when someone makes a comment such as that you've no dignity or self-respect?

Actually, one might question whether such a remark calls for any response at all. Examine what you did or did not do that elicited this remark in the first place and evaluate it for yourself. But this question calls for a social answer that demonstrates that you do have self-respect; give a thoughtful, mature response, after some thought. One good way to respond to any criticism is “Thank you for pointing out this imperfection in my personality. I will try to work on it.” Another tactic might be a humorous remark, such as “It’s difficult for me to see anything remarkable about myself, except of course my humility.” Or give a philosophical response: “I respect the complexity of the universe, and my small place in it.”