Among the several allergens cloned and expressed from Aspergillus fumigatus, Asp f 4 is a major one associated with allergic bronchopulmonary aspergillosis (ABPA). The structure-function relationship of allergens is important in understanding the immunopathogenesis, diagnosis, and treatment of allergic diseases. These include the epitopes, conformational or linear, deletion of the N or C terminus or both N and C termini, and glycosylation or nonglycosylation, all of which affect immune responses. Similarly, the role of cysteine residues present in allergens may yield useful information regarding the conformational structure of allergens and the immunoglobulin E (IgE) epitope interaction. Such information may help in developing new strategies towards immunotherapy. In order to define the role of cysteine in the interaction of the antibody with Asp f 4, we have constructed mutants by selectively deleting cysteine residues from the C-terminal region of the Asp f 4. Immunological evaluation of these engineered recombinant constructs was conducted by using sera from patients with ABPA, Aspergillus skin test-positive asthmatics, and healthy controls. The results demonstrate strong IgE binding with Asp f 4 and two truncated mutants, Asp f 4(1-234) (amino acids [aa] 1 to 234) and Asp f 4(1-241) (aa 1 to 241), while another mutant, Asp f 4(1-196) (aa 1 to 196), showed reactivity with fewer patients. The result suggests that deletion of cysteines and the alteration of IgE epitopes at the C-terminal end resulted in conformational changes, which may have a potential role in the immunomodulation of the disease.