The kinetics of the antibody response in cattle have been analyzed and the following results were obtained: A single administration of proteins to cattle given intravenously or as alum‐precipitated antigen evoked a concurrent synthesis of γM‐ and γG‐antibodies as reflected in the serum 7 days following immunization. Employing the antigen binding capacity method for assaying antibody activity, the γG‐antibody collected 7 days after immunization exceeded by several times that of the γM‐class. The γG‐antibody response as reflected in serum and colostrum three weeks after primary stimulation showed a marked increase when compared with their respective antibody levels formed earlier. The colostral 19 S antibody, composed of 19 S γG‐globulin and the easily discernible γM‐component was also increased and surpassed by several times the γM‐antibody level in a 3‐weeks serum. When colostrum was fed to the newborn calf, the γG‐antibody appeared to be absorbed quantitatively by the intestinal tract, whereas the high molecular weight globulins (19 S) were transmitted much less efficiently. This might indicate that some sort of selective absorption occurred between different classes of immunoglobulins. Kinetic studies of the secondary response demonstrated that in cattle serum an anamnestic reaction exists for γM‐antibody as well as for γG‐antibody. The dissociation of the 19 S molecules from serum and colostrum by reduction and alkylation produces little effect on the antigen binding capacity of the monomer subunits obtained. The kinetic characteristics of serum and colostral 19 S‐ and γG‐antibodies were studied by measurement of the association rates. γM‐Antibodies present in serum 7 days following antigenic challenge had a higher affinity for protein antigens than γG‐antibody molecules. There is considerable evidence, however, that the affinity of the γG‐antibody for the antigenic determinants increased progressively with time after immunization. Copyright © 1968, Wiley Blackwell. All rights reserved
