The Relationship Between Health and Cell-Mediated Immunity Measured in Ecology: Phytohaemagglutinin Skin-Swelling Test Mirrors Blood Cellular Composition

The phytohaemagglutinin (PHA) skin-swelling test, a common ecological method for measuring immune responsiveness, uses a subcutaneous injection of PHA to induce an inflammatory response with metrically measurable swelling. Both the immunological basis of this response and the role of pre-exposure physiology remain poorly understood, however, preventing predictions of costs and benefits of immunity investments revealed by the test and their relationship to life-history traits. In this study, we measure PHA-triggered tissue swelling, cellular infiltration into inflamed tissue, and haematological changes in zebra finches Taeniopygia guttata, with the aim of identifying those cell types involved in activation of PHA-induced skin swelling. We show that the inflammation underlying the swelling response is dependent on initial haematological state and alters the composition of circulating leukocytes. Injection of PHA decreased heterophil and monocyte frequency and increased basophil frequency, suggesting changes in leukocyte trafficking and haematopoiesis. Higher initial basophil and lymphocyte frequencies in peripheral blood were associated with increased swelling. High pre-treatment blood lymphocyte levels led to intense lymphocyte migration into inflamed tissue, while high initial basophil levels resulted in lower cellular infiltration. Our results show two possible directions in the association between pre-exposure health and swelling response: (i) a lymphocyte-based pathway that leads to higher responsiveness in high-quality healthy individuals capable of higher energy and resource investment, or (ii) higher responsiveness in diseased individuals with initially elevated basophil levels and pre-activated immunity. Haematological investigation, therefore, facilitates interpretation of PHA skin-swelling test results in ecological immunology, explaining ca. 20 % of variation in skin-swelling data. (C) 2015 Wiley Periodicals, Inc.