Immune system efficacy is a very important element of human physiology, and is perceived as one of the best measures of an organism’s biological condition. Any defect in the immune function leads to inadequate protection and poses a significant risk to survival. According to the ICHH, morphological traits that are sexually dimorphic or related to sexual ornamentation may signal biological quality, and therefore could also reflect good immune quality. To test the hypothesis that androgen-dependent male ornamentation is also an honest signal of biological quality, we analyzed various parameters of immunity in relationship to different androgen levels in healthy adult males. In the well-nourished Western society studied here, we found very limited evidence for a relationship between some androgens and adaptive immunity. Our results do not confirm the hypothesis that androgens exert immunosuppressive effect. Below we discuss our results in details, categorized by the immune topic.

When controlling for age and BMI, none of the measured innate immune parameters were related to androgen concentrations. Our results are in contrast to the in vitro study by Marin et al.42, which documented an inhibitory effect of testosterone on neutrophil function. The experimental studies on mice with knockout of androgen receptors show, however, that these mice have functional defects of neutrophils including the lower production of proinflammatory cytokines, but retain the normal capacity of phagocytosis and ROS production43. We obtained the similar result in men i.e. no effect of androgens on neutrophils functions.

Our study corroborates the correlational study of Prall and Muhlenbien22 on complement activity and saliva DHEA concentrations. Although the data documenting a potential down-regulatory role of androgens in innate immunity are very limited, the evidence from correlational studies indicates that innate immune factors are independent of serum androgen concentrations.

We found that when controlling for age and BMI, androgens were not associated with the total number of T and B lymphocytes, i.e. major cells involved in adaptive cell-mediated and humoral immunity, respectively. A functional test of lymphocytes showed that androgen concentrations were not related either to lymphocyte proliferation after mitogenic stimulation or to total IgA and IgG immunoglobulin levels. Interestingly, the strength of the immune response to the influenza vaccine was positively predicted by the level of fT and marginally significantly by DHT. This indicates that hormones can affect rather the function of lymphocytes (in response to a stimulus) than their total number in blood.

The majority of human and animal in vitro studies which focused on quantitative analyses of lymphocytes have shown a negative impact of testosterone on lymphocyte proliferation44,45,46 or antibody production47. A limited experimental human study conducted on transsexual Caucasian women found no effect of testosterone administration on IgG levels48. The results of correlational studies, however, are inconclusive. Whereas some reported results similar to ours, i.e. observing no association between immunoglobulins and testosterone levels49, others found a positive relationship50. It is worth noting that in contrast to the study on Filipino men51, our study, as well as those of van Anders49 and Giltay et al.48 (the latter only on women) were conducted in well-nourished, affluent societies. These differences may suggest the influence of ecological factors on immune-testosterone interaction. In a correlational study linking lymphocyte function with androgens other than testosterone, Prall and Muehlenbien22 found that saliva DHEA level was negatively correlated with lymphocyte proliferative response. Because of the small sample size (only 20 men) and no controlling for participant age, adiposity, or health status, the conclusions of Prall and Muehlenbien should be, however, treated with caution.

In general, our results on the baseline immune parameters and androgens are compatible with the meta-analysis findings on many species by Foo et al.51. They showed that in contrast to experimental studies, there was no significant overall correlation between testosterone and immune function in correlational studies.

There are also inconclusive data for the relationship between androgen concentrations and the ability to produce specific antibodies post-vaccination. Our results show that men with higher levels of fT produce more specific anti-influenza antibodies in response vaccination. This finding is similar to those obtained by Rantala et al.20, who found a positive relationship between total testosterone levels and immune response to hepatitis B vaccination. The opposite finding for fT and response to influenza vaccination was reported by Furman et al.21. Their men sample, however, was small (N = 34), and the median age of participants was 63 years. Age is an important factor in immune response to vaccination it is because the response tends to decrease with age. In Furman et al.21, only 10 out of 37 men (27%) responded positively to the influenza vaccine by showing seroconversion. In our study 75.3% of the vaccinated men were seroconverted, what is similar to the rate obtained in healthy young people in another study (68%–84%)52. Furthermore, Furman et al.21 did not control for adiposity, which may influence the immune response to the influenza vaccine53.

Due to the small sample of men who were also vaccinated against tetanus in our study, we did not present the response analyses for this antigen. It is worth noting, however, that although nonsignificant (p = 0.11), the relationship between fT and the response to tetanus vaccination was in the same direction (R = 0.27) as that of the influenza vaccine.

There are a few possible explanations of higher immune reactivity in more androgenic men (with higher level of fT). Firstly, it is possible that in energetically replete populations (as in the case of our study), men with high genetic quality can afford to invest in more costly adaptive immune arm and at the same time in high androgens production. Such explanation agrees with the hypothesis that only individuals with high immune quality are able to incur the immunosuppressive effect of testosterone. Secondly, the proportional marginal viability cost that is associated with high testosterone level is lower for individuals with higher immune quality. According to Getty et al.54 the experimental and correlational studies might only reflect absolute cost, whereas it is proportional marginal fitness cost (i.e. in relationship to benefits) that might be crucial for an individual ability to develop sexual signal. Thirdly, because the immune activation in response to infection is related to down-regulation of testosterone levels, higher testosterone in men can be consider as a signal of better physiological condition (including immunity)55. Finally, if testosterone is not absolutely immunosuppressive (but rather immunomodulatory), high genetic quality individuals can invest more in maintenance both in high level of viability (reflected by immune efficiency) and in fecundity (active form of androgens).

Our results also indicate that to test the hormonal basis of the immunocompetence hypothesis, the immunological challenge (such as vaccination) may be more informative than baseline immune parameters or functional tests performed on isolated immune cells (such as neutrophil phagocytic uptake or lymphocyte proliferative response). It is because of the nature of immune system network function, when the basal levels of immune parameters occurring in healthy individuals do not necessarily reflect reactivity of immune cells in response to a real infection.

Overall, we did not confirm the ICHH as postulated by Folstad and Karter2. There is, however, one problem with this hypothesis. The positive, negative, or lack of relationship between androgens and immune function do not need to undermine the ICHH54. In the case of a positive relationship, one can assume that individuals with high testosterone levels can also afford high immunity, and in the case of a negative relationship, that healthy men are trading off between both. The majority of research validating the ICHH has been performed on birds, but results of animal studies are also difficult to interpret because of a lack of “normal range” definition for immunological parameters in healthy individuals. Higher levels of measured leukocytes or immunoglobulins may be interpreted as a higher immune potential (better immunity) or as a result of outgoing infection (weaker immunity)6. In the case of individuals living in Western societies, recent data do not support either the ICHH or the positive relationship between the traits perceived as attractive and immune functioning. Foo et al.56 for instance, found no relationship between components of men’s facial attractiveness and selected innate immunity factors. This indicates that testosterone-dependent facial masculinity does not have to signal the immune effectiveness.

In the light of previous results, the main question is why androgens exert different effects on immune cells in vitro and in vivo. Possibilities include molecular mechanisms responsible for local androgen conversion by immune cells, synthesis of specific enzymes, or patterns of sex hormones receptors expressed on immune cells. Firstly, it is possible that in vivo, testosterone can exert indirect effects on immune cells after local conversion by immune cells to estrogenic metabolites and binding to estrogen receptors57,58. It is well-known that estrogens up-regulate immune functions, and this may be one reason for the lack of immunosuppressive effect of testosterone observed in our study. Secondly, ARs are expressed by various immune cells and play an essential role in androgen-associated immune regulation (see15). Studies on mice lacking androgen receptors59 or experimental human studies60 have shown that the comprehensive impact of hormones on immune response depends not only on hormone concentrations but also on qualitative and quantitative expression profiles of specific immune-cell receptors present in various tissues. This evidence suggests that individual differences in the expression of ARs on various immune cells can explain why the same androgen levels may exert effects of varying magnitude on immunity. Lastly, the effect of AR signaling differs in vitro and in vivo. Yang et al.61 for instance, showed that AR activation exerted an inhibitory effect on prostatic epithelial cell proliferation in vitro, whereas in vivo, the activation had a stimulatory effect. According to above evidence, to better understand androgen-immune interaction, further investigation of ICHH assumptions should include not only hormone levels but also specific receptor expression patterns and enzyme synthesis by immune cells which convert androgens into other metabolites.

The second problem is related to different physiological or ecological factors influencing immune-androgen interaction. Both animal and human studies indicate that immunity development in early life can be crucial. In this respect, McDade et al.24 suggests three potential elements: “(a) the availability of nutritional resources, (b) the intensity of pathogen exposure and (c) signals of extrinsic mortality risk”. The combination of these factors may lead to distinct immunomodulatory effects by androgens. There are reports showing trade-offs between different fitness components when resources are limited24,62. Some animal studies showed that food availability can influence the relationship between testosterone levels and immunity63 and that leptin may prevent the immunosuppressive role of testosterone64. Other factors which may affect testosterone-immune interactions are stress20 and living conditions65. Trumble et al.65 found that cell-mediated response to mitogens was inversely related to testosterone levels in Tsimane men aged 40–89 from an energy-limited subsistence population, which faces a high pathogen burden and is therefore immunologically stressed.

Finally, it should be noted that the majority of empirical studies (both experimental and correlational) are mainly designed to measure absolute cost of “signals” (androgens level) which will might be not sufficient to define viability-fertility (i.e. immune-hormones) trade-offs54. According to Getty et al.54 costly signals should be considered as an investment instead of handicaps, and we should avoid the assumptions that individuals who express higher sexual signals must have “wasted” more viability to develop them than lower quality individuals with lower expression of sexual signals. The ICHH prediction is based on an absolute cost associated with the levels of expression sexual signals, whereas it is the marginal fitness cost and benefits (not easily measurable in empirical studies) that might be crucial54.

Despite some limitations, the strength of our study is derived from four factors: (a) A relatively large sample of men in their reproductive age, which is important when testing evolutionary hypotheses; (b) a variety of immune biomarkers evaluated; (c) four different androgens studied; (d) controlling for general health status using medical biomarkers.

It is also worth noting that although our study is mainly correlational, it is the first study testing the relationship between different androgens concentration and many other immune functions simultaneously. This means that in contrast to previous studies, our conclusions are about the general immune-androgens in vivo interactions that are based on multiple immune parameters.

In summary, we found that fT and DHT are probably the hormones most relevant for studying immune parameters potentially affected by androgens. These two reactive forms of androgens positively influence adaptive immunity, i.e. the strength of the response to the influenza vaccine, and appear to have no effect on humoral or cell-mediated innate immunity. This finding contrasts with the immunosuppressive function of androgens observed in vivo. The relationship between androgens and immune function is, however, complex, and probably restricted only to particular immune parameters and does not apply to all. Therefore we suggest treating androgens as immunomodulators rather than immunosuppressants, as supported by the important role of ecological factors in moderating the relationship between immunity and androgens. Further investigations regarding the association between serum androgen concentrations, ARs, and various immune functions are required.