The aetiology of Pigeon Fanciers’ Lung (PFL) is believed to include immune complex formation between inhaled pigeon antigens and antibodies generated against them. However it is unclear why some fanciers are asymptomatic despite the presence of high levels of anti-avian antigen antibodies in their serum. In this study we investigated whether qualitative differences in specific antibodies might contribute to disease. IgG responses among pigeon fanciers were determined by ELISA and the functional affinity of IgG1 and IgG2 against a range of pigeon antigens was determined by inhibition ELISA and Isothermal Titration Calorimetry (ITC). The median titres of IgG1 and IgG2 against all the pigeon antigens tested was higher in asymptomatic than symptomatic fanciers and these differences were significant for anti-pigeon serum IgG1 (P = 0.04), anti-fresh pigeon droppings (PDF) IgG2 (P = 0.028), anti-old pigeon droppings (PDO) IgG2 (P = 0.04) and anti-pigeon intestinal scrapings IgG2 (P = 0.03). The functional affinity of IgG1 and IgG2 against PDO was higher in symptomatic individuals (P = 0.006 and P = 0.002, respectively) whilst the functional affinity of anti-PDF IgG2 was also significantly higher in these patients (P ≤ 0.001). Symptomatic fanciers were also significantly more likely to have a high reaction enthalphy (ΔH) as measured by ITC and thus had higher affinity antibodies against PDO (P = 0.044). This data confirms previous studies showing that the magnitude alone of the antibody response to pigeon antigens cannot determine the presence of PFL, but that antibody affinity may be important. ITC is a rapid method of measuring antibody affinity and has diagnostic potential in PFL, and may be of use in other situations where antibody affinity is important.