Figure 1.

Schematic presentation of Immunoglobulin A (IgA) forms, subclasses, and distribution in humans. Distribution of IgA subclasses (IgA1 and IgA2) in external secretions of healthy adults. The relative proportions varying in different locations. Immunohistochemical studies and results from short- term culture experiments of human tissues supported the above-described distribution of the form of IgA (polymeric or monomeric) and the isotype (IgA1 or IgA2) in several fluids that parallels the distribution of cells in various tissues and organs. Measurements of antigen-specific antibodies in individual external secretions mirrored the distribution of IgA1- or IgA2- producing cells in the corresponding mucosal tissues.