126 IMMUNITY AND INFECTION 
The part played by side-chains of the second order, a^gkitinins 
and precipitins, in immunity is not well understood. Their relation 
to immunity is less clear than the relation of antitoxin to immunity. 
Side-chain of the Third Order.— Nutritive substances of large mole- 
cular aggregation may require considerable modification to fit them 
for cellular assimilation. Such substances are removed from the 
blood stream and bound to the cell by side-chains of the third order. 
They are then acted upon by an enzyme (complement) which is 
also present in the blood stream. It will be seen that both the nutri- 
tive element and a digestive enzyme circulate in the blood, but that 
no reaction occurs between them until they are both united by a 
side-chain of the third order, which must therefore consist essentially 
of two combining groups. One of these, the cytophilic group or 
haptophore, unites specifically with the nutritive element. The other 
combining or haptophore group, the complementophilic group, unites 
with the enzyme or complement which is present in the blood stream. 
Side-chains of the third order are called amboceptors because they 
possess two combining groups. An excessive irritation of a cell by 
a substance capable of uniting with the cytophilic group of a side- 
chain of the third order will lead to overproduction and elimination 
of these side-chains precisely as toxins lead to an overproduction of 
side-chains of the first order (antitoxin formation). The side-chains 
of the third order, furthermore, exhibit specificity for the substance 
which led to their overproduction, just as antitoxins exhibit specificity 
for their homologous toxin. It has been shown that the zymophoric 
group of a side-chain of the second order is permanently a part of the 
structure. The complement, which is analogous to the zymophore 
group of the second order, is not attached to a side-chain of the third 
order until the cytophilic group of the latter has combined with its 
antigen. The zymophore group of the second order side-chain is 
readily destroyed and it cannot be replaced. The zymophoric group 
of the third order side-chain is not an integral part of the structure, 
and it can be introduced under appropriate conditions. 
Third order side-chains or amboceptors are cytolysins. Those 
specific for bacteria are called hacteriolysins; those specific for blood 
cells are called hemolysins; and those specific for the cells of various 
tissues or organs are called cytolysins. Bacteriolysins appear to be the 
most important of the specific antibodies in effecting the destruction 
and removal of pathogenic bacteria which have invaded the tissues. 
Available evidence indicates that most of the bacteria of the "oppor- 
tunist" group do not stimulate lysin formation. 
The activity of the lysins, according to the Ehrlich theory, depends 
on the union of non-specific complement and a specific antigen by the 
specific amboceptor. A union of antigen and amboceptor may take 
place according to this theory in the absence of complement, but a 
union of antigen and complement cannot take place in the absence of 
amboceptor. The amboceptor, like other haptophore groups is rela- 
