156 ANTIGENS AND THE TECHNIC OF SERUM REACTIONS 
stabile component "alexin." Pfeiffer^ then showed that the destruc- 
tive action of normal sera could be increased many fold aboAC its 
original level for a specific orf/anism if that organism were repeatedly 
injected into an animal in sublethal, but gradually increased doses. 
The serum of such an animal would still destroy only moderate num- 
bers of heterologous bacteria, but relatively great numbers of the homo- 
logous bacteria. This observation opened the way for the highly 
important study of active acquired immunity against bacteria. Pfeiffer 
observed that heating immune sera to 50° to 56° C. for one-half hoin- 
destroyed their bactericidal properties, precisely as Nuttall had found 
that natural, non-specific bactericidal properties were destroyed under 
similar conditions. Bordet- then discovered that the addition of a 
small amount of unheated blood serum from a non-immune animal 
would "reactivate" the heated inactive immune serum and restore 
its bactericidal power to its original level. These experiments col- 
lectively demonstrated clearly that: 
1. Normal sera had an inherent but limited destructive action upon 
a variety of bacteria. 
2. That this destructive or bactericidal action could be greatly 
increased for specific organisms through repeated injections of sub- 
lethal doses of them .3 
3. That both normal and immune sera lost their bactericidal proper- 
ties by heating them to 55° C. for one-half hour. 
4. That immune sera would regain their specific bactericidal power 
if a small amount of fresh normal blood serum from a non-immune 
animal were added to them,^ 
Bordet^ showed similarly that the red blood cells of an alien animal 
were also destroyed to a limited degree by the serum of a normal 
animal, but that this destruction could be greatly increased for specific 
erythrocytes if they were repeatedly injected into an experimental 
animal. The blood serum becomes specifically hemolytic. Here 
again Bordet*^ found that heating an immune serum to 55° C. for 
thirty minutes destro.yed its activity, but that a small amount of 
fresh serum from a non-iipmune animal (whose serum per se would 
not dissolve the homologous cells) would reactivate the serum. Thus, 
both specific bacteriolytic sera and specific hemolytic sera must con- 
tain two distinct components— a thermostabile component resisting 
an exposure to 55° C. for one-half hour and contained only in the 
immune serum, and a thermolabile component destroyed or inacti- 
vated at 55° C, which is present both in active immune bacteriolytic 
and hemolytic sera, and also in normal sera. To the thermolabile 
1 Ztschr. f. Hyg., 1894, 18, 1; 1895, 19, 75. 
2 Ann. Inst. Pasteur, 1898, 12, 688; 1899, 13, 273. 
3 Presumablj^ leaving the original non-specific bactericidal power at its initial level 
for all except the specific organism, and possibly for closely related forms. 
* Moxter: Centralbl. f. Bakteriol., 1899, 26, 344. 
' Loc. cit. 
« Ann. Inst. Pasteur, 1898, 12, 688. 
