562 IMMUNOLOGY 



divided material in the air, organic and inorganic compounds in 

 food, water and the various substances with which one comes in 

 contact, it is not surprising that allergic disease is common and 

 that the exciting agents are numerous. In those cases in which 

 allergy apparently occurs on the first ingestion of food, Zinsser, 

 Enders and Fothcrgill (1939) suggest that active sensitization may 

 have occurred in utero by the passage of the exciting agent from 

 the mother through the placenta. In fact they state that there is 

 little doubt that both active and passive sensitization of an infant 

 can take place before birth. For some reason, however, the child 

 is not often born hypersensitive. 



Effect of Heredity on Age of Onset. — Heredity apparently in- 

 fluences the time of onset of allergy. Those with a bilateral in- 

 heritance are all sensitive by the age of forty, 36 per cent being 

 sensitive before the age of five, while hypersensitive individuals 

 with a unilateral and with negative family histories of allergy 

 become sensitive as a rule at relatively later periods of life. 



Facts About Reagin. — The antibodies or reagins which are 

 responsible for the P-K reaction have been studied extensively. 

 A few of the more important things that have been learned about 

 them may be summarized as follows : 



1. When a serum containing reagin is mixed with the specific 

 atopen, the reagin titer is reduced or removed, but the capacity 

 of the exciting agent (atopen) to elicit skin reactions remains 

 unchanged. 



2. The reaction between reagin and atopen does not lead to the 

 formation of a visible precipitate. 



3. AVhile a very small amount of a serum containing reagin 

 can passively sensitize the human skin, it cannot passively sensi- 

 tize the skin of the lower animals. 



4. Reagin is apparently more susceptible to heat than pre- 

 cipitins or protein sensitizers. 



5. Many individuals clinically desensitized by the repeated in- 

 jection of extracts of an atopen responsible for hay fever or 

 asthma remain skin sensitive. 



6. In some individuals positive skin tests can be obtained re- 

 peatedly in the same area, 



