The perils and prospects of using phytohaemagglutinin in evolutionary ecology

Several techniques are available for quantifying the vertebrate immune response, information that is particularly useful for understanding the contribution of immunity to the evolution of life-history strategies. The most widely used is the phytohaemagglutinin (PHA) skin-swelling technique, which is usually regarded as an index of acquired immunity. However, our understanding of the effects of PHA in skin is poor, despite the fact that it has implications for what the test can tell us about immune activity. As we discuss here, a recent study by Martin and colleagues on the response to PHA at the cellular level in wild birds has highlighted the relative extent to which PHA-induced swelling, as most commonly applied, measures innate immunity versus acquired immunity.