Normal serum from male and female rabbits aged 1 o weeks or more possessed complement-fixing activity when mixed with rabbit semen, washed or epididymal spermatozoa, and the non-dialysable fraction of seminal plasma obtained from semen, but not with seminal plasma obtained from a vasectomized buck. Sera from younger animals did not have this activity. The active fraction in seminal plasma originated in the spermatozoa. Absorption with epididymal spermatozoa reduced the activity in two sera considerably, and slightly in a third. Other tests, i.e. passive cutaneous anaphylaxis (PCA) in the guinea-pig, precipitation tests, and agglutination or immobilization of spermatozoa in vitro, did not indicate the presence of antibody-like activity in normal sera.
Five rabbits were injected twice with rabbit semen in adjuvant. The serum of one, and possibly another, of these rabbits possessed complement-fixing activity against seminal plasma from a vasectomized buck, and also showed a slight rise in titre against epididymal spermatozoa after the injections. Positive PCA against spermatozoa and seminal plasma could not be demonstrated in sera from any of the five rabbits, although precipitins against seminal plasma, and agglutinating and immobilizing antibodies against epididymal and ejaculated spermatozoa were found in sera from two of the rabbits. Agglutination and immobilization of spermatozoa were probably caused by antibodies against seminal plasma.
The presence of natural complement-fixing activity in serum against spermatozoa, and the failure to induce significant amounts of antibodies against spermatozoa as judged by the absence of PCA and precipitins, or by a definite rise in complement-fixing titre, is discussed in relation to the characteristics of the immunological tests employed. The results are also briefly discussed in relation to the clonal selection theory of antibody formation, and to other natural antibody systems.