Thank you for your inquiry.
Actually, systemic cold urticaria (generalized urticaria occurring upon exposure to cold) was first reported in 1984 (see abstract copied below). We have personally seen several cases of cold urticaria occurring underneath clothed areas. It usually occurs on the trunk and extremities but I'm not aware of any predisposition to involve the axilla or groin. And I am not sure that the highest counts of mast cells are found in these areas. Other studies have found the highest count in the abdominal skin for example:
Mast Cell Population in Human Skin*
Another found the count higher in the distal arm and leg than in the proximal areas (J Clin Pathol March 2005, 58(3:285)
So in summary the presentation of your patient is in my experience typical for systemic cold urticarial, and from my review of the relative density of mast cells in the skin I was not able to confirm an increased density in the axilla and groin. However I could have missed a report and we would love a followup if there is confirmation of an elevated mast cell count in the groin and axilla.
Thank you again for your inquiry and we hope this response is helpful to you.
J Allergy Clin Immunol. 1984 Apr;73(4):453-6.
Unusual cold-induced disorders: cold-dependent dermatographism and systemic cold urticaria.
Two patients are described in whom new cold-induced disorders were defined. The patients presented with histories suggesting either dermatographism or typical cold urticaria; however, standard tests for each disorder failed to reproduce the patients' symptoms. One patient had dermatographism that was observable only upon chilling the skin subsequent to scratching it. The reaction was associated with detectable elevation of venous histamine levels coincident with the development of urticaria. The second patient had generalized urticaria that was induced by systemic rather than local cold challenge. The reaction was anaphylactoid in nature and was associated with systemic elevation of histamine levels. This patient was also mildly dermatographic, and skin reactivity was markedly augmented during an episode of generalized hives. These cold-dependent disorders should be included in the differential diagnosis of patients with histories suggestive of cold urticaria with or without a dermatographic component. Further evaluation is indicated when the standard tests for either of these disorders are negative.
Phil Lieberman, M.D.