Antigenic competition has been studied in C3H/HeJ mice between determinants of the multichain synthetic polypeptide antigens, (Phe, G)-A--L, (T, G)-Pro--L and (Phe, G)-Pro--L. Since these mice did not respond detectably t o (T, G) and only very weakly t o A--L, the relevant immunopotent regions involved were Pro --L and (Phe, G). Thus, these animals generated a good response t o the (Phe, G) determinant, when given as (Phe, G) -A --L , and a good response t o Pro --L, when administered either as such or as (T, G) -Pro --L. However, when the immunogen was (Phe, G) -Pro --L, the antibody response was mainly towards Pro--L, whereas a very poor response was obtained to the (Phe, C) specificity. This is interpreted as antigenic competition between P r o --L and (Phe, G), with Pro--L the dominant and (Phe, G) the suppressed determinant.
Competition was also demonstrated with mixtures of (T, C) -P r o --L and (Phe, G)-A --L . Competition with the mixed antigens was dependent on the relative amounts of the immunogens in the mixtures. Thus, a molar excess of (T, G) -Pro --L over (Phe, (3)-A --L of 5 t o 1 suppressed the response t o (Phe, G), but an excess of (Phe, G)-A--L of 5 t o 1 suppressed the of 25 t o 1 caused suppression of the anti-Pro --L response. With equimolar mixtures, an optimal primary response to both (Phe, C) and P r o --L was observed. Where competition had occurred in the primary response, n o priming was detected for a secondary response. Under certain conditions immunological memory could be suppressed by competition even though an apparently normal primary response had occurred, suggesting that memory is more susceptible t o suppression by competition than is the primary response.
Thc ability of an excess of (Phe, G) -A --L t o suppress the response t o P r o --L could be reproduced completely with A --L alone. Thus, a molecule that is very poorly immunogenic, but which functions as a carrier for haptens, can be successful in competition.