How does optimism suppress immunity? Evaluation of three affective pathways.

Dispositional optimism, defined as generalized positive outcome expectancies, has been associated with positive health consequences in a number of studies. More optimistic men recovered better from coronary artery bypass surgery (Fitzgerald, Tennen, Affleck, & Pransky, 1993; Scheier et al. 1999), more optimistic men and women had lower blood pressure during daily life (Raikkonen, Matthews, Flory, Owens, & Gump, 1999), and more optimistic women infected with both HIV and human papilloma virus had better immunity (Byrnes et al., 1998). These positive effects on physical health mirror optimism’s consistently beneficial effects on mental health. However, four reports of optimism’s effects on the immune system – two experimental and two naturalistic – have also identified circumstances under which optimism is related to lower immune parameters that might have negative health consequences. First, an experimental study manipulated the degree of control that the participant had over a loud noise stressor (Sieber et al., 1992). Optimism’s effects differed depending on the condition, such that more optimism was associated with higher natural killer cell cytotoxicity (NKCC) when control was present but with lower NKCC when it was not. A second experimental study assigned some participants to perform a mental arithmetic task that became more difficult with better performance so that mastery was never attained; others did not perform the task (Segerstrom, Castaneda, & Spencer, 2003). Among those who did not perform the task, higher optimism associated with larger delayed-type hypersensitivity (DTH) responses, which measure in vivo cellular immunity; among those who did perform the task, higher optimism associated with smaller responses. Third, a prospective, correlational study examined the effects of optimism and naturalistic stressor duration (F. Cohen et al., 1999). Higher optimism associated with higher T cell counts (CD8+CD11b+) when stressors lasted less than one week, but with lower T cell counts (CD8+CD11b−) and NKCC when stressors lasted longer than one week. Finally, two prospective correlational studies associated higher optimism with higher T cell counts (CD4+ lymphocytes) and larger DTH responses among students experiencing low conflict between social and academic goals, but with lower T cell counts and smaller DTH responses among students with high conflict (Segerstrom, 2001). The pattern of results across these four reports suggests that optimism associates with higher immune parameters when stressors are straightforward (time-limited, controllable, responsive to coping efforts, not conflicting) but lower immune parameters when stressors are difficult (prolonged, uncontrollable, unresponsive to coping efforts, conflicting). Affect was cited as a likely mediator for these effects. F. Cohen et al. (1999) and Sieber et al. (1992) posited that although optimists are typically less distressed than pessimists, optimists who encounter difficult stressors might be more disappointed, distressed, and demoralized than their pessimistic counterparts because such stressors violate their positive expectancies (c.f., Tennen & Affleck, 1987). In this disappointment model, optimism leads to negative affect that subsequently decreases immune parameters. In contrast, the engagement or persistence model states that difficult stressors elicit different responses from optimists and pessimists: optimists reengage, putting forth more effort to overcome the stressors, whereas pessimists disengage (Carver & Scheier, 1998; Segerstrom, 2001; Segerstrom et al., 2003). Affect is a potential mediator in the engagement model as well, but the engagement model involves more narrowly specified categories of potential affective mediators, as opposed to the diffuse negative affect of the disappointment model. First, engaged affect (e.g., interested, determined) is associated with larger cardiovascular and immune responses to laboratory stressors, suggesting that, although positively valenced, engaged affect might have negative physiological consequences (S. Cohen et al., 2000; Maier, Waldstein, & Synowski, 2003). Second, fatigue could reflect the prolonged self-regulatory efforts of optimists facing difficult stressors. Studies of “ego depletion” or “self-control depletion” find that difficult self-regulatory tasks decrease persistence at subsequent problem-solving tasks and sometimes increase perceived fatigue but not other kinds of affect (Baumeister, Bratslavsky, Muraven, & Tice, 1998). The present study examines the effects of optimism on cellular immunity under conditions of high and low goal conflict in first-year law students. The lack of time to pursue both academic and social goals (e.g., spending time with family and friends) is a major difficulty during the first year of law school (Segerstrom, 1996). Furthermore, students who do not geographically relocate are more likely than those who do to have goals relating to maintaining and especially finding time for existing relationships, suggesting that they experience this difficulty more acutely as a function of remaining in their usual social context (Segerstrom, 2005). Optimism was predicted to have a positive relationship with immunity among students experiencing little conflict (i.e., students who relocated) but a negative relationship with immunity among students experiencing more conflict (i.e., students who did not relocate). Support for affective mediation consistent with the engagement model was predicted to be better than that for the disappointment model because (1) negative affect and negative affectivity have not accounted for the effects of optimism on immunity in some previous investigations (F. Cohen et al., 1999; Segerstrom, 2001); (2) potentially disappointing events such as health relapse have not increased distress for people with higher premorbid optimism, suggesting that optimists are not at risk for increased negative affect following expectancy violation (Litt, Tennen, Affleck, & Klock, 1992; Helgeson, 2003; Stanton & Snider, 1993); and (3) in the mental arithmetic study, the effects of conscientiousness, a personality factor related to goal engagement, paralleled those of optimism, where as neuroticism, a personality factor related to distress, did not (Segerstrom et al., 2003). The present study used the DTH test to measure the cellular immune response in the skin, one of the sites in the body where pathogens are most commonly encountered. In the major steps of the DTH response, antigen-presenting cells in the skin stimulate T cells to produce cytokines, which recruit and activate effector cells. The result of the migration of these effector cells into the skin is redness and swelling or induration in the skin that can be measured reliably as an index of how robust the immune response is. Larger indurations, therefore, indicate more robust cellular immune responses (Janeway & Travers, 1997).