SUNBURN 517 



the same region which, though present in much less amount, have rather 

 similar absorption spectra. Thus attempts to analyze the erythemal 

 spectrum, even when carried out as carefully as Mitchell has done, cannot 

 lead to very definite conclusions. So, while nucleoprotcin has been indi- 

 cated as the light absorber for the photochemical reaction initiating the 

 principal theme schematized in Fig. 13-11, even this very basic postulate 

 must be open to question (particularly in light of recent studies, Blum 

 et al, 1954). 



In the diagram (Fig. 13-11) the immediate result of the photochemical 

 reaction in the epidermal cells is described rather vaguely as injury. 

 Among other substances elaborated by these cells is a dilator substance 

 which brings about the vasodilation of the minute vessels of the dermis, 

 and is grossly manifested as erythema. An increase in dilator substance 

 has been demonstrated in skin (Nathan and Sack, 1922; Ellinger, 1930) 

 and blood (Laurens and Kolnitz, 1940) of animals that have been exposed 

 to ultraviolet radiation, but the identity of the dilator substance con- 

 cerned in the erythema of sunburn has not been clearly shown. Lewis 

 and Zotterman (1926) suggested that it was histamine or a histamine-like 

 H-substance, and this view was once rather widely accepted. However, 

 Percival and Scott (1931) found that the response of the skin to histamine 

 pricks is not altered by the presence of the erythema of sunburn, and this 

 is also true for skin which has been exposed to ultraviolet radiation but 

 has not yet developed erythema (see Blum and Terus, 1946a). While the 

 initial erythema of sunburn is rigidly restricted to the area exposed, after 

 severe doses of ultraviolet radiation a red flare may develop which 

 extends outward from the exposed area. This usually appears only after 

 about 24 hr. The flare is irregular and differs in general appearance from 

 the initial sharply limited erythema. Lewis and Zotterman (1926) called 

 attention to the resemblance of this flare to that which results from prick- 

 ing histamine into the skin, but, in the latter case, the flare appears 

 almost immediately, and they do not adequatel}^ explain the delay in the 

 case of sunburn. According to Lewis' hypothesis (1927), the histamine 

 flare is the result of antidromic impulses in afferent nerves and in the case 

 of sunburn it seems likely that stimulation of these nerves might result 

 from injury to the fibers themselves or to surrounding tissues. The flare 

 is most pronounced when the erythema is induced by longer wave lengths 

 of the erythemal spectrum, suggesting that the dermis itself, which is 

 reached by those wave lengths, may be the locus of this effect. Krogh 

 (1929) concluded that there must be at least two dilator substances con- 

 cerned in sunburn, and, if erythema and flare are separate entities medi- 

 ated by different substances, this should be true. 



The whole (luestion of the nature of the dilator substance is complicated 

 by the finding that a dilator substance is produced by photochemical 

 changes in the corneum (Rottier, 1952, 1953; Rottier and Mullink, 1952). 



