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This type of lesion could be many different issues. I am not sure it is related to her immune-deficiency, unless she has a dermatological immune mediated disease.
With a red sore on her paw, it could be a focal infection (bacterial, yeast) or a tumor or an area of trauma or an aggravated bite. There are other possibilities. This is probably something, if it persists and/or is worsening, given her age you must have your veterinarian evaluate since tumor is on the list as a potential.
In the meantime until you can get her in to be seen, I recommend cleanse the area well. You can do a paw bath (not full body bath unless you would like) with either a cleansing dog shampoo (not medicated over-the-counter shampoo, though) or you could use Selsun Blue original as made for people (but do not let her eat the shampoo). Then you may safely apply neosporin or other antibiotic ointment to this two to three time daily. She will probably lick this off. If it is highly irritating to her (licking a lot) then you may need to put her in an Elizabethan collar (the lampshade collar) or place a sock over the paw but be careful not to bandage it tightly. This may buy you some time, but unless this is a traumatic lesion, I do not think it will resolve on its own, and your veterinarian will need to do an exam.
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