What structure is primarily involved in the condition known as hives?

The condition known as hives, also referred to as urticaria, is characterized by elevated lesions on the skin. These lesions appear as raised welts that may vary in size and are often accompanied by itching. The elevated nature of these lesions is a result of an allergic reaction or other triggers that cause the release of histamine and other mediators, leading to swelling of the skin's surface.

The other choices pertain to different skin conditions: localized pus collection refers to abscesses, which are pockets of infection; the upper layer of the skin references the epidermis, which can be affected but is not specific to hives; and fluid-filled sacs within the skin pertain to blisters or vesicles, which are not characteristics of hives. Thus, the correct answer reflects the distinctive raised welts that define this allergic skin reaction.