324 Lesions Produced by the Bite of the Black-fly 
Third, lytic agents in the blood serum may play the chief role 
in the liberation of the toxic agent from its non-toxic combination. 
An immune individual would then be one whose immunity was not 
the positive one of antibody formation, but the negative immunity 
of failure to metabolize. An immune lesion in such a case might 
be conceived as presenting no eosinophilia, since no toxin is liberated. 
If the liberation of the toxin is dependent upon lytic agents present 
in the serum rather than in any cellular elements, a rational explana¬ 
tion would be available for the apparent results (subject to con¬ 
firmation) of the experiment with sensitive and immune sera. In 
this experiment it will be recalled that the sensitive serum seemed to 
bring out the toxicity of the ground flies, and the serum itself seemed 
even to contain some of the dissolved or liberated toxin. The 
slowness with which a lesion develops in the case of the black-fly 
bite supports the view of the initial lack of toxicity of the injected 
material. The entire absence of early subjective symptoms, such 
as pain, burning, etc., is further evidence for this view. It would 
appear as if no reaction occurred until lysis of an originally non¬ 
toxic substance had begun. Regarding the toxin itself as the chemo- 
tactic agent which attracts eosinophiles, its liberation in the lytic 
process and diffusion through the blood stream attracts the cells 
in question to the point at which it is being liberated. Arriving 
upon the scene, these cells assist in its neutralization. 
The last view presented is the one to which the author inclines 
as the one which most adequately explains the phenomena. 
A fourth view is that the initial injection of a foreign protein by 
the fly (i.e.,with the first bite) sensitizes the body to that protein. 
Its subsequent injection at any point in the skin gives rise to a 
local expression of systematic sensitization. Such local sensitization 
reactions have been described by Arthus and Breton, by Ham¬ 
burger and Pollack and by Cowie. The description of such a lesion 
given by the first named authors, in the rabbit, however, does not 
suggest, histopathologieally at least, a strong resemblance to that 
of the black-fly. Such an explanation of many insect urticaria 
deserves further investigation, however, and may align them under 
cutaneous expressions of anaphylaxis to a foreign protein injected 
by the insect. Depending on the chemical nature of the protein 
injected, a specific chcmotactic reaction like eosinophilia may or 
may not occur. Viewed in this light the development of immunity 
to insect bites assumes a place in the larger problem of anaphylaxis. 
