Yes, you can (as long as it's no longer your turn).
You can not, however, use a swift action and an immediate action on the same turn.
Also, if you use an immediate action when it's not your turn (as in your example above), you will lose the opportunity to use a swift action on your next turn.
For clarity, you can only use one swift or immediate action per round. Your round begins the moment after your initiative count and runs through the end of your next turn.
For example, there are four combatants You, A, B and C. You go third in initiative order after A and B but before C. A and B take their turns and now it's your turn and you do your thing. Once you are done doing your turn, your first round ends. The moment C begins their actions, your second round begins. Between now and when you are finished with your next set of actions, you may make either one swift or one immediate action. So, lets say in this round you get knocked below 0 hp so you activate your Hero's Defiance and save yourself. On your turn, you cannot also use Lay On Hands on yourself as a swift action, since you have effectively spent your swift action earlier in the round by using the immediate action. Once C begins to act again, you regain the ability to use swift/immediate actions.
I hope that helps.
Using an immediate action on your turn is the same as using a swift action and counts as your swift action for that turn. You cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when it is not currently your turn (effectively, using an immediate action before your turn is equivalent to using your swift action for the coming turn). You also cannot use an immediate action if you are flat-footed.
If you use a swift action on your turn, then as soon as your turn is over, you are able to use an immediate action.