Table 1.

Four explanations for sex differences in immunity (columns, see text) framed in terms of expectations in the ‘caring’ sex (generally females), and expected alignment with immune trade‐offs (rows) with implications for senescence (final row)

Trade‐off	Quantitative differences	Qualitative differences	Effect of transfer of antibodies	Effect of pregnancy
Having an immune systema	More expenditure	Either	Either	Either
Maintaining an immune systema	More expenditure	Either	Either	Either
Discriminating to trigger a response or not (favouring false positives vs. false negatives)	Possibly more triggering (more ‘sensitive’, Figure 3a)	Undefined (for Th1/Th2 contrast), or less triggering, that is less sensitive (for discrimination contrast)	More trigger (more sensitive)	Either
Magnitude of the triggered responsea	Larger response	Smaller inflammatory response (for Th1/Th2 contrast) and immune effector response (for discrimination contrast)	Either (but likely smaller)	Larger response to offset females spending time immunosuppressed to tolerate a foetus
Tolerating the infection (without reducing burden) versus nota	Possibly greater tolerance to offset greater responses?	Greater tolerance (under Th1 vs. Th2 contrast) or either (discrimination contrast)	Either	Either
Resistance, that is excluding infectiona	Larger investment	Either	Either	Either
Implications for immune effects on survival in the caring sex at late ages	Greater immunopathology and immune memory depletion; greater survival in the face of late life infections, unless memory depletion has reached problematic levels	Less (for Th1/Th2 contrast) or more (for discrimination contrast) immunopathology; and less memory so less defence against infection for the discrimination contrast	More autoimmunity, immunopathology	More autoimmunity, more immunopathology; assuming that change is in magnitude rather than regulation

^avs. expending resources on another aspect of life history (survival, fertility).