antibodies generated by the female. early research in 1899, the discovery of the existence of antibodies against sperm was made independently both by serge metchnikoff of the pasteur institute and by nobel prize laureate karl landsteiner. in 1929, the first recorded attempt at immunocontraception was made by morris baskin, clinical director of the denver maternal hygiene committee. in this trial 20 women who were known to have at least 1 prior pregnancy were injected with their husband's semen, and no conception was recorded in 1 year of observation of these couples. a united states patent (number 2103240) was issued in 1937 for this approach as a contraceptive, but no product for widespread consumption ever came from this approach. renewed interest throughout the 1990s, there was a resurgence of research in immunocontraception targeting sperm with the hope of developing a contraceptive vaccine for human use. unlike earlier research which explored the contraceptive effect of immune responses to whole sperm cells, contemporary research has focused on searching for specific molecular antigens that are involved with sperm function. antigens that have been identified as potential targets for immunocontraception include the sperm-specific peptides or proteins adam, ldh-c4, sp10, sp56, p10g, fertilization antigen 1 (fa-1), sp17, sob2, a9d, cd52, ylp12, eppin, catsper, izumo, sperm associated antigen 9 (spag9), 80 kilodalton human sperm antigen (80 kda hsa), and nuclear autoantigenic sperm protein (tnasp). early primate trials had mixed results. one study examined the sperm-specific isozyme of human lactate dehydrogenase (ldh-c4) combined with a t-cell epitope to create a synthetic peptide that acted as a more potent chimeric antigen. vaccination of female baboons with this synthetic peptide resulted in a reduced fertility in the trial. however, a second study that examined vaccination of female macaque monkeys with the same synthetic peptide did not find reduced fertility. since then, a study examining vaccination based on an epididymal protease inhibitor (eppin) in male macaque monkeys demonstrated that vaccination against sperm antigens could be an effective, reversible contraceptive in male primates. while 4 of 6 control monkeys impregnated females during the trial, none of the 7 monkeys included in the trial that were vaccinated against eppin impregnated females, and 4 of these 7 vaccinated monkeys recovered their fertility within a year and a half of observation after the trial. this illustrated that not only could sperm immunocontraception be effective, but it could have several advantages over zona pellucida vaccines. for instance, sperm vaccines could be used by males, in addition to females. additionally, while there are relatively few glycoproteins in the zona pellucida and thus relatively few target antigens for zona pellucida vaccines, more than a dozen prospective target antigens for the inhibition of sperm function have been identified. this relative abundance of prospective target antigens makes the prospects of a multivalent vaccine better for sperm vaccines. a study which examined the use of one such multivalent vaccine in female macaque monkeys found that the monkeys produced antibodies against all antigens included in the vaccine, suggesting the efficacy of the multivalent approach. finally, while there has been autoimmune ovarian pathogenesis found in some trials using zona pellucida vaccines, anti-sperm antibodies are not likely to have adverse health effects, since anti-sperm antibodies are produced by up to 70% of men who have had vasectomies, and there has been much investigation into possible adverse health side-effects of the vasectomy procedure. passive immunity a vaccine induces active immunity when antigens are injected into an animal that cause it to produce desired antibodies itself. in passive immunity the desired antibody titers are achieved by injecting antibodies directly into an animal. the efficacy of such an approach for immunocontraception was demonstrated as early as the 1970s with antibodies against zonae pellucidae in mice during the investigation of the mechanism by which such antibodies inhibited fertility. because the variability of individual immune response is an obstacle to bringing contraceptive vaccines to market, there has been research into the approach of contraception through passive immunization as an alternative that would be of less duration, but be closer to market. research done using phage display technology on lymphocytes from immunoinfertile men led to the isolation, characterization, and synthesis of specific antibodies that inhibit fertility by acting against several of the known sperm antigens. this detailed molecular knowledge of antisperm antibodies may be of use in the development of a passive immunocontraceptive product. gamete outcome human chorionic gonadotropin most of the research into immunity that inhibits gamete outcome has focused on human chorionic gonadotropin (hcg). hcg is not necessary for fertilization, but is secreted by embryos shortly thereafter. therefore, immunity against hcg does not prevent fertilization. however, it was found that anti-hcg antibodies prevent marmoset embryos from implanting in the endometrium of their mother's uterus. the main function of hcg is to sustain the ovarian corpus luteum during pregnancy past the time it would normally decay as part of the regular menstrual cycle. for the first 7–9 weeks in humans, the corpus luteum secretes the progesterone necessary to maintain the viability of the endometrium for pregnancy. therefore, immunity against hcg during this time frame would function as an abortifacient, as confirmed by experiments in baboons. in the scientific literature the more inclusive term "birth control vaccine" rather than "contraceptive vaccine" is used to refer to hcg vaccines. clinical trials research begun in the 1970s led to clinical trials in humans of a hcg birth control vaccine. a phase i (safety) clinical trial examined 15 women from clinics in helsinki, finland, uppsala, sweden, bahia, brazil, and santiago, chile with a vaccine formed by conjugating the beta subunit of hcg with a tetanus toxoid. the women had previously had tubal ligations. in the trial the immune response was reversible and no significant health issues were found. this was followed by another phase i trial in 1977-1978 examining previously sterilized women at 5 institutions in india with a more potent vaccine that combined the beta subunit of hcg with the alpha subunit of ovine luteinizing hormone to form a heterospecies dimer conjugated with both tetanus toxoid and diphtheria toxoid. the multiple carriers were used because it was found that a small percentage of women acquired carrier-specific immunosuppression due to repeated injection of conjugates with the same carrier. this more potent version of the vaccine was used in a phase ii (efficacy) trial during 1991-1993 conducted at 3 locations: the all india institute of medical sciences, safdarjung hospital in new delhi, and the post graduate institute of medical education and research in chandigarh. primary immunization consisted of 3 injections at 6 week intervals, and 148 women known to be previously fertile completed primary immunization. all women generated antibodies against hcg, but only 119 (80%) generated antibody titers clearly above 50 ng/ml, which was the estimated level for efficacy. blood samples were taken twice a month, and booster injections were given when antibody titers declined below 50 ng/ml in women who wished to continue using the vaccine. at the completion of the study after 1224 menstrual cycles observed, only 1 pregnancy occurred in a woman having an antibody titer level above 50 ng/ml, and 26 pregnancies occurred among women who had titers below 50 ng/ml. application to cancer therapy following these clinical trails of hcg vaccination as a birth control method, hcg was discovered to be expressed in certain kinds of malignant neoplasms, including breast cancer, adenocarcinoma of the prostate, progressive vulvar carcinoma, carcinoma of the bladder, pancreatic adenocarcinoma, cervical carcinoma, gastric carcinoma, squamous-cell carcinoma of the oral cavity and oropharynx, lung carcinoma, and colorectal cancer. therefore, immunity against hcg has applications such as imaging of cancer cells, selective delivery of cytotoxic compounds to tumor cells, and in at least one case, direct therapeutic effect by preventing establishment, inhibiting the growth, and causing the necrosis of tumors. this has led to interest in developing hcg vaccines specifically for cancer treatment. ongoing research the vaccine tested in the phase ii clinical trial in india did not proceed further because it produced antibody titers of 50 ng/ml for at least 3 months duration in only 60% of women in the trial. ongoing research in hcg birth control vaccines has focused on improving immunogenicity. a vaccine in which the beta subunit of hcg is fused to the b subunit of escherichia coli heat-labile enterotoxin has been effective in laboratory mice. it has been approved by the indian national review committee on genetic manipulation and is being produced for pre-clinical toxicology testing. if it is determined to be safe, it is planned for clinical trials. wildlife control immunocontraception is one of the few alternatives to lethal methods for the direct control of wildlife populations. while there was research into the use of hormonal contraception for wildlife control as early as the 1950s that produced pharmacologically effective products, they all proved to be ineffective for wildlife control for a variety of practical reasons. field trials of immunocontraception in wildlife, on the other hand, established that contraceptive vaccines could be delivered remotely by capture gun, were safe to use in pregnant animals, were reversible, and induced long-lasting infertility, overcoming these practical limitations. one concern about the use of hormonal contraceptives in general, but especially in wildlife, is that the sex steroid hormones that are used are easily passed, often via the food chain, from animal to animal. this can lead to unintended ecological consequences. for instance, fish exposed to treated human sewage effluents were found to have concentrations of the synthetic hormone levonorgestrel in blood plasma higher than those found in humans taking hormonal contraceptives. because the antigens used in contraceptive vaccines are protein, not steroids, they are not easily passed from animal to animal without loss of function.