Avian Immunology and Serological Assays


Allison German
Article on Avian Aspergillosis from Allison German in the new Veterinary section:


No abstract. First paragraph: Work in avian species has suggested differences in immune system characteristics from mammalian counterparts. It is recognised that bird immunoglobulin is classed as IgY, a more primitive Ig than mammalian IgG, representative of IgG and IgE prior to their split into the individual classes recognised in mammals (Horton and Ratcliffe, 1996). This is noteworthy, since elevations in both IgG and IgE have been useful in the diagnosis of human aspergillosis. Research into the human Ig subclass response has shown greater increases in IgG1 in chronic pulmonary aspergilloma patients compared to ABPA (transient antigen exposure). PA patients who also had elevated IgE showed a greater rise in IgG4 levels compared to other patients. IgG4 is thought to be an indicator of chronic inflammatory disease and is possibly a marker of protection against allergy (Tomee et al, 1996). The authors proposed that elevated IgG1 production may indicate Th1 stimulation due to chronic fungal exposure in PA patients, whereas elevations in IgG4 may result from a Th2 response in patients with PA also producing IgE. The subclass response was also investigated by Igea et al (1993) using ELISA, demonstrating elevations in IgG2 and IgG4 alongside IgE in allergic patients. The IgG4 elevations may suggest a role for the intestine in sensitisation. However, given the increases in IgG4 in non-allergic individuals, elevations in this subclass may be protective or pathogenic, depending on the circumstances. Basophil sensitisation by the IgG4 subclass may contribute to allergy or block Type I hypersensitivity. However, the role of cytokines, IgE and the Th1/Th2 cell ratio is much more complex than that implied above, with Il-4, produced by activated Th2 cells, able to stimulate class switch to IgG1 and IgE (Brostoff and Hall, 1996)Martinez-Quesada and others (1993) investigated the immunoresponse in pigeons by looking at CIE, indirect haemagglutination and ELISA results on serial bleeds of A. fumigatus inoculated birds. Two peaks were noted in the immune response: an early rise (maximal by 2 weeks post-immunisation) correlating with IgM, and a later rise (maximal by 63 days) correlating to IgG. Immunoglobulin (Ig) levels fell with cessation of antigenic stimulation, and showed a rapid memory response following a booster injection at 270 days.Cellular immunity may be more important than humoral immune responses, as suggested by the development of generalised disease in human aspergillosis patients with compromised cellular immunity (Bardana et al 1975). Precipitating antibodies were found in 90-100% of patients with aspergilloma, whereas only 57% of invasive/disseminated cases gave a positive result. Both Bardana and Tomee also acknowledged a rise in serum IgA in all disease syndromes studied. Knutsen et al (1994) noted that elevations in anti-Af IgG, IgE and IgA occurred during ABPA in patients with cystic fibrosis. With remission, IgE levels decreased markedly, though IgG and IgA levels tended to persist. This elevation in IgA indicates a potential role for mucosal immunity, which warrants further investigation.