METHODS IN MAMMALIAN IMMUNOGENETICS 349 



change occurs in the system. The discrimination of two phases came about largely as 

 a result of observations of systems in which the first step occurred without the second, 

 and in which the second could be promoted by making some nonspecific addition to or 

 change in the system. For example, in some systems, although antigen-antibody 

 unions occur at low salt concentrations or in the cold, visible changes do not follow 

 until salt is added or the system is warmed. Red cells combine specifically with their 

 corresponding antibodies at low temperatures, but they fix complement and lyse in 

 relatively short times in the presence of complement only when the system is warmed. 

 Such observations tend to distinguish between a first stage of antigen-antibody com- 

 binations and a derivative second phase which may or may not follow depending on 

 the conditions of the test; nevertheless, in many systems the ultimate effect is generally 

 believed to be the result of the continued operation of the same mechanisms as are 

 involved in the initial antigen-antibody unions. For example, a visible precipitate is 

 generally believed to form as bivalent antibodies combined with polyvalent antigens in 

 a continuous process, the antibody molecules providing bridges in a lattice forming the 

 developing aggregate. The degree to which nonspecific secondary interactions may 

 enter into serologic reactions is often debatable and probably depends on the particular 

 system under test. For example, in some systems of cellular agglutination there is 

 reason to believe that changes in cellular membranes consequent upon the initial 

 antibody attachments at isolated sites may reduce the tendency of cells to repel each 

 other, and therefore lead secondarily to aggregation. Hemolysis clearly depends on 

 attacks on the antibody-sensitized cell by nonspecific components of normal serum, 

 leading only secondarily to cell lysis. Many reactions in vivo, such as anaphylaxis, also 

 depend on secondary consequences of the initial antigen-antibody combinations 

 occurring in tissues. 



When a system has been defined for a particular test (such as saline agglutination) , 

 it is often found that some of the antibodies capable of uniting specifically with the 

 antigen are unable to promote the second, visible stage of the test reaction. Such 

 antibodies are often described as incomplete antibodies. A number of methods are 

 available for their detection; for example, they may block the antigen by combining 

 with it, so that access to the relevant sites on the antigen is prevented for complete 

 antibodies later added to the system. Or they may promote aggregation in other 

 media, though they fail to do so in saline. They may sensitize cells for hemolysis but 

 not promote agglutination, or they may combine with cells and render them subject to 

 aggregation by other antibodies directed against the antibody globulin molecules 

 themselves. Or they may provide a vehicle for passive transfer of tissue sensitization, 

 so that, for example, if they are injected into the skin of an insensitive individual they 

 confer a transient local sensitivity to their corresponding antigen at that skin site. In 

 precipitating systems, multivalence of both antigen and antibody has been mentioned 

 as a necessary condition for the appearance of a visible aggregate. Failure of an anti- 

 body preparation to produce an aggregate is sometimes the occasion for the use of the 

 term "univalent" with reference to the antibody. Often the term is an unfortunate one; 



