A variable prevalence of ASA has been reported depending on the specificity and sensitivity of the test used for their detection and on the screened population. The first assays to be utilized were indirect tests detecting biological activities of ASA in serum and seminal plasma, i.e., sperm agglutination techniques and complement-dependent sperm immobilization/cytotoxicity techniques. Subsequently, widespread acceptance has been gained for antiglobulins-based tests used to detect antibodies coated to the surface of ejaculated spermatozoa, including the mixed antiglobulin reaction (MAR) test and the immunobead binding test (IBT). They reveal the percentage of antibody-coated spermatozoa, the Ig-isotype, and grossly the regional specificity of ASA. Old multicentric comparative studies [7, 54] showed that all these tests determine in large measure the same antibody specificities for surface antigens, but with different sensitivity, which is lower for complement-dependent sperm-immobilization/cytotoxicity techniques.

In epidemiologic studies, serum sperm-agglutinating activity ranged from 8.1 to 30.3 % in unselected men with infertile marriages [13, 21, 26, 40], but at low titers it was also reported up to 10 % of control sera [26]. When stricter criteria were used (i.e., the occurrence of sperm-immobilizing activity in addition to high titers of sperm-agglutinating activity in the serum and/or sperm-agglutinating activity in seminal plasma, indicating an excess of free antibodies in the semen), the prevalence of ASA in men with infertile marriages ranged from 4.6 to 5.7 % [21, 38]. Direct tests (MAR or IBT) gave positive results (>10 or >20 %) in 7.6–12.9 % of unselected infertile patients [11, 21, 38, 47], but highly positive results (≥50 %) were restricted to 5–6 % of patients [11, 21, 47]. In the most recent and largest multicentric survey, including 1794 consecutive subfertile couples, a positive IgG-MAR ≥10 % was detected in 7 % of men and a positive IgG-MAR ≥50 % was detected in 3 % of men [32].

Although a higher prevalence of ASA has been reported in some clinical conditions, including genital tract obstructions and infections, especially by Chlamydia trachomatis, testicular trauma, torsion, and surgery (see Chap. 8), only acquired genital tract obstructions represent a well-established risk factor: in vasectomized men the prevalence of ASA ranges from 33 to 74 % [5, 19, 29, 37], with their persistence in 38–60 % following successful vasovasostomy [5, 37].

A causal link between ASA and fertility impairment, although suggested by epidemiologic studies, can only be proven by the association of the occurrence and the degree of sperm autoimmunization with a reduced pregnancy rate, independently from semen parameters and other clinical characteristics.

In retrospective studies, when the degree of sperm-autoimmunization was taken into account, it exhibited a significant inverse correlation with the incidence of spontaneous pregnancies. In an old report by Rumke et al. [46], during a 10-year follow-up of 254 infertile men with serum sperm-agglutinating activity (SAA), the titer of SAA was inversely correlated with the occurrence of spontaneous pregnancies. Notably, restricting the analysis to normozoospermic men, no pregnancy was observed with very high titer of serum SAA (≥1:1024), a low (15.8 %) pregnancy rate (PR) with titers ranging from 1:32 to 1:512, and a high PR (48.4 %) with titers <1:32. Ayvaliotis et al. [4] reported that in 108 infertile couples where the male exhibited a direct IBT >10 %, and the female was treated for other factors leading to impaired reproduction, PR was significantly higher when IBT was <50 % than when it was >50 % (43.4 % vs. 21.8 %) during a follow-up of at least 18 months. The difference in PR was ever of greater significance in a subgroup of 35 couples, where no other cause of infertility was found (15.3 % vs. 66.7 %). Abshagen et al. [1] reported that in 157 infertile couples with a direct IBT >10 %, cumulative spontaneous PR over 6 years was high (~50 %) when IBT was <50 %, lower (~30 %) when IBT was 50–90 %, and very low (~15 %) when IBT was >90 %, independently from the IgG-class (IgG and/or IgA). A significant inverse correlation between the degree of sperm autoimmunization and PR was also found in a follow-up study of 216 men after vasovasostomy by Meinertz et al. [39]. While no pregnancy was observed in a median period of ~4 years in men where all spermatozoa were antibody-coated at MAR test, in association with a high titer of serum SAA, pregnancy occurred in 64.3 % of couples with a less degree of sperm-autoimmunization. While in this study only a prevalent IgA autoimmunization was associated to a reduced fertility, a major role of IgA was not found in another study on vasovasostomized men by Matson et al. [37].

Altogether, these observations suggest that ASA represent a relative, rather than absolute, cause of infertility and the degree of fertility impairment appears to be related to the extent of sperm-autoimmunization. Accordingly, a threshold of 50 % positivity at MAR or IBT test has been established by WHO [56] for a degree of sperm-autoimmunization which might be clinically relevant.

However, the best evidence for a causal link between ASA and fertility impairment would be provided by prospective studies comparing the occurrence of natural pregnancies in men with and without ASA. Unfortunately this evidence is difficult to obtain: (1) the low incidence of sperm autoimmunization in unselected infertile couples requires multicentric studies including a large number of infertile couples (or, for the best evidence, couples without history of infertility) and a large number of observed cycles; (2) the inter-individual variability of semen parameters, not related to the presence of ASA, makes it very difficult to obtain a study- and a control-population, homogeneous for semen quality; and (3) the inter-couples variability in other clinical characteristics. Owing to these limitations, little information along with conflicting results has been produced by scanty prospective studies so far reported [13, 18, 55]. In the most recent and largest study [32], a positive IgG-MAR test ≥50 %, detected in 3 % of 1794 patients with infertile marriages, reduced, albeit not significantly, the probability of pregnancies during a 1-year follow-up. At the multivariate analysis, including semen parameters and other clinical characteristics, a positive MAR test ≥50 % did not contribute to the prediction of spontaneous pregnancy.

Therefore, the proof of a causal link between ASA and infertility has not yet been produced by the evidence-based medicine.

Only ASA directed towards surface antigens have a physiopathological and clinical significance in the male immunological infertility, because subsuperficial antigens cannot be exposed to antibodies by living cells along the male genital tract.