I should have pointed out more clearly that the depth of the wound is the most important factor.

If it doesn't break all the way through the skin, its no big deal.
The problems start if it goes all the way through to the underlying tissues. I should also point out that the amount of blood you see is no indication of depth. If you can see fat or other tissue coming up out of the hole, it is deep enough to take very seriously.
I get bitten on a fairly regular basis due to work (dogs, cats, you name it). Immunity over time is not really a factor. I don't treat the superficial wounds with other than simple cleaning, but I have also had a number of very deep punctures as well. Fortunately I have immediate access to antibiotics. Even with antibiotics, I have had times where a single puncture wound to my finger caused so much swelling that I could not use my hand for 2-3 days.
Another factor to think about is keeping tabs on where the cat goes. There are public health laws concerning bite wounds. Even though rabies is highly unlikely, cats with unknown vaccination historys are usually quarantined for about ten days after the bite.
By the way, I don't want to sound negative about the cat. I really hope its owner is located or it finds a nice new home. I just don't want anyone seriously hurt by taking bite wounds too lightly.