122 IMMUNITY AND INFECTION 
a number of chemical affinities or receptors, for convenience of discus- 
sion designated as "side-chains" or "haptines," which are the means of 
attaching to the cell by chemical union, the essential nutritive sub- 
stances preparatory to their assimilation. When the particular food 
attached to the cell by chemical affinity— anchored by the side-chain, to 
use Ehrlich's terminology— has been assimilated, more of the same kind 
of food is removed from the blood stream and attached to the cell, in 
accordance with its normal physiological requirement. The cell, 
acting through its side-chain, does not exhibit discrimination between 
nutritive substances and irritating or harmful substances which may 
accidentally possess the same combining affinity for the cell. Conse- 
sequently, when poisonous substances possessing chemical affinities 
similar to those of the normal food substances circulate in the blood 
stream, they may become attached to the cell in place of the normal 
physiological nutrients. The anchoring of these poisonous substances, 
unlike the attachment of normal nutrient substances, is followed by 
damage to the cell, or, in extreme cases, by the death of the cell.^ If 
the cell is not actually killed by the presence of the toxic substance 
acting upon it through the side-chains, it is irritated, as it were, and 
the toxic substance imposes a twofold burden upon the cell— loss of 
the side-chains to which it is attached and which are essential to 
maintain the nutrition of the cell, and greater or lesser damage to the 
function of the cell, due to toxic inhibition of its normal activities. A 
cell cannot disembarrass itself of the poison, nor can it assimilate it. 
It can, however, throw off the side-chain with the poison still firmly 
united to it chemically ; the extruded poison cannot enter into chemical 
combination with other cells possessing the same chemical affinity, 
for it is already attached to a side-chain. Its combining power is 
saturated. 
Side-chains are a necessity to the cell, however; without them the. 
cell would starve. Consequently the cell regenerates new side-chains 
of precisely the same kind to replace those thro\\Ti off after being bound 
to non-assimilable substances. If enough of the soluble poison or 
toxin circulates in the blood stream, this process of union of toxin to 
the cell by its side-chains and its expulsion from the cell with the side- 
chains attached to it is so frequently repeated that the cell regenerates 
side-chains in excess of the normal requirements, in accordance with 
the Weigert theory of overproduction. This casting off of supernu- 
merary side-chains is important. Were they not cast off the cell 
would be vulnerable to toxin in direct proportion to the extra number 
of side-chains, which would furnish extra bonds for its attachment. 
As the cast-off side-chains circulate in the blood stream, however, they 
are an element of protection to the cell, for they retain their original 
combining power for the toxin and unite with it and neutralize it as it 
1 If the toxic material circulates in the blood stream but does not become attached 
to the body cells, it is harmless to the host, according to this theory, and the host is 
naturally immune. 
