Specific Agglutination and Precipitation
Authors: Eagle Harry 
Published: January 2023

Summary and Conclusions
Shaking, an increase in the concentration of antibody, and a decrease in the total volume, all cause a more rapid aggregation of bacteria, red cells, protein or lipoid by the corresponding antiserum. Up to a certain optimum value, an increase in the amount of antigen, in the electrolyte concentration, or in temperature accelerates specific aggregation; further increase may cause retardation.
It has been suggested by various investigators that specific aggregation is due to the formation of a sensitizing film of antibody globulin around the antigen. According to this theory, the velocity of aggregation rests largely on two factors: (1) the number of impacts between the sensitized particles, and (2) the proportion of impacts which are effective, i.e., result in cohesion. With the exception of temperature, the effect of the various factors cited above is satisfactorily explained by such a theory. To consider them individually, it is believed that: Increase in antibody accelerates aggregation by forming a more complete film of sensitizing antibody globulin around the antigen particle. The fact that a maximum velocity is attained, beyond which further increase in antibody is ineffective, is thought to be due to the formation of a complete film of sensitizing protein.Shaking acts by increasing the number of impacts between the sensitized particles, i.e., antigen already combined with antibody. It is particularly effective in facilitating the grouping of small aggregates into larger, more readily visible clumps.A decrease in total volume accelerates aggregation by decreasing the average space between antigen particles, and thus increasing the total number of impacts. With too large a total volume, even maximally sensitized antigen particles may fail to form visible aggregates.The accelerating action of electrolytes is due to the fact that they discharge the ionized protein film. In the absence of salt, this mutually repellent charge suffices to prevent cohesion. Electrolytes depress this charge below a “critical” level at which aggregation begins. The velocity of aggregation increases up to an optimum electrolyte concentration at which the antigen particle is almost completely discharged; further addition of electrolyte is ineffective, and may cause an unexplained retardation.The effect of temperature upon the velocity of aggregation is as yet unexplained.An increase in antigen causes, first, an acceleration of aggregation due to the increase number of particles. With an excess of antigen, however, this is more than counterbalanced by the decreased sensitization (less antibody per unit antigen). There is thus an intermediate zone of optimum antigen concentration.
The observation that the velocity of every type of antigenantibody aggregation, whether bacterial, red cell, protein or lipoid, is similarly affected by any given change in the environmental conditions, furnishes additional evidence for the conception that all antigen-antibody reactions involving aggregation are fundamentally similar. As stated in the introduction, the common factor in all these reactions is considered to be the sensitizing film of antibody globulin.