Complement fixing activities of normal mammalian sera for homologous and heterologous sperm

The species of the natural anti-sperm antibodies found in normal mammals have been assessed using complement fixation tests. Heated rabbit sera showed complement fixing activity with both rabbit and guinea pig sperm, indicating that rabbit natural anti-sperm antibodies do not show clear cut species specificity in these interactions. On average, rabbit sera reacted more strongly with rabbit sperm that with guinea pig sperm but the difference fell short of statistical significance (P < 0.05) and there was considerable variation between sera in their anti-sperm activities. There were, however, clear species differences between rabbit and guinea pig sera in that heated guinea pig sera showed significantly weaker anti-sperm activity than heated rabbit sera (P < 0.005). Mouse vas sperm showed greater complement fixing activities (in combination with mouse serum) than did epididymal sperm (P = 0.05). These results are discussed in relation to possible functions of natural anti-sperm antibodies.