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Routinely used formal saline fixation reduces the number of demonstrable mast cells in human skin by up to 30% compared with paired specimens fixed in Carnoys medium. Using metachromatic (toluidene blue), orthochromatic (alcian blue/safranin), enzymatic (chloroacetate esterase reaction) and immunofluorescence (berberine and fluorescein conjugated avidin) staining techniques, mast cells were demonstrated and quantified. Alcian blue/safranin and fluorescein-conjugated avidin were both superior to the other staining methods used. We recommend the use of Carnoys medium fixed tissue stained with either alcian blue/safranin or conjugated avidin for optimal visualization and assessment of mast cells in human skin.

p53 immunoreactivity was examined in 132 cutaneous non-melanoma tumours from renal transplant recipients and in 114 histologically matched specimens from immunocompetent individuals. Skin lesions examined included 52 viral warts, 50 dysplastic keratoses, 51 intraepidermal carcinomas (IEC), 50 invasive squamous cell carcinomas (SCC) and 43 basal cell carcinomas (BCC). Overall, 51% (51/101) pre-malignant skin lesions and 45% (42/93) non-melanoma skin cancers (NMSC) showed p53 immunoreactivity, with extensive (> 50% cells positive) p53 staining in 27% (27/101) of pre-malignant and 20% (19/93) of malignant lesions. 17% (9/52) viral warts showed p53 immunoreactivity, but this was limited to focal or basal p53 staining. p53 immunoreactivity in all tumours was less in transplant than in non-transplant patients and this reached statistical significance for SCCs (p = 0.03).

Alteration in the major histocompatibility complex (MHC) antigen expression by cutaneous tumours may enable them to escape host defence mechanisms and to invade surrounding tissue. Immunohistochemical studies in a wide range of epidermally derived tumours demonstrated expression by keratinocytes of the class II molecule HLA-DR in squamous cell carcinoma (SCC) (2 of 8 cases) and keratoacanthoma (KA) (2 of 7 cases). Additionally, HLA-DP and DQ were expressed by single cases of SCC and KA, although, unlike the widespread distribution of DR, DP and DQ, were only present on keratinocytes adjacent to the inflammatory infiltrate. Therefore, keratinocytes in cutaneous tumours, like carcinoma cells of the colon and breast, may express class II MHC antigens during tumour growth. Beta-2-microglobulin (B2M), an invariant MHC class I marker, was absent in all cases of basal cell carcinoma. Variable loss of B2M was observed in squamous cell carcinoma, Bowen's disease and actinic keratoses, suggesting reduced B2M expression by dysplastic cells. However, the variability in B2M staining both between and within diagnostic categories restricts it's immunodiagnostic usefulness.

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