These spots of blood under the skin (purpura) are caused by vasculitis. They do not turn white with pressure (non-blanchable). In this particular case, the purpura are associated with an underlying disorder affecting the structure of the blood vessel walls (collagen-vascular disorder).

Inflammation of the blood vessels (vasculitis) may be caused when antibodies that have attached to antigens in the blood (immune complexes), attach to the blood vessel walls. These purplish spots can be felt in the skin. They do not turn white (blanch) when pressed. As the condition progresses, they may become larger and more bruise-like (ecchymotic), and some may develop central ulceration or necrosis (tissue death).

These red (erythematous), hive-like (urticarial) spots (plaques) are caused by inflammation of the blood vessels (urticarial vasculitis) and do not change over a 24-hour period. They may or my not turn white (blanch) with pressure.