A simple method for measuring the amount of immunoglobulin A secreted onto the skin surface.

We developed a simple method for measuring the amount of the secretory form of immunoglobulin A (sIgA) present in sweat. A small disk (10 x 10 mm) made of cellulose membrane was attached to the skin surface for periods of 1 to 24 h. SIgA was absorbed to the membrane and accumulated during the period of application. Enzyme immunoassay using anti-sIgA and antisecretory component (SC) antibodies revealed distinct dots on the disk that corresponded to the eccrine excretory ducts. A densitograph was used to determine the number and density of the dots, thus obtaining the amount of sIgA excreted to the surface of the skin (per mm2). The amount of skin sIgA excreted differed inter-individually as well as intra-individually. That is, it varied according to the region of the skin, and its distribution roughly reflected that of the sweat ducts. SIgA excretion was maintained at a certain level, regardless of the increased sweating produced by either heat or exercise, which raised the output of sweat 3- to 15-fold. Immunohistochemical studies revealed that fewer glandular cells expressed SC in their cytoplasm as the amount of sIgA decreased. Such an independence of the excretion of sIgA from that of sweat may be necessary to the local immune defenses of the skin.