Hello Cowgirl,
There are several skin conditions/diseases that can produce necrosis or necrotic looking wounds, which complicates the proper diagnosis of a Brown Recluse bite. Lyme Disease, Cellulitis, Necrotizing fasciitis, Gangrene, are among them.
The Brown Recluse spider is equally venomous no matter where it finds its home. Bite wound progression will vary depending on the amount of spider the venom injects, and the location of the bite wound. Fatty areas are more likely to experience the most serious wounds, and yes the immune system is a factor.
Brown recluses consider homes to be an ideal habitat, and they like to hide in walls, ceilings, boxes and other undisturbed places when resting or breeding. As long as a homeowner does nothing to remove the spider, they are unlikely to ever want to permanently leave.
Let me know if you have any further questions.
Thanks,
Tom
www.Brown-Recluse.com