ABSTRACT
An assay of β-glucan was materialized by the discovery of Factor G, a blood coagulation factor in the amebocyte lysate of the horseshoe crab that is extremely sensitive to β-glucan (Morita et al., 1981). The amebocytes are hemocytes containing a set of coagulation enzymes, of which endotoxin was thought to be the sole potent activator. Thus, the lysate was originally used to detect endotoxin (Levin et al., 1968) and was named the Limulus amebocyte lysate test, or Limulus test, after the American horseshoe crab, Limulus polyphemus. The test was believed to be specific for gramnegative bacteria until about 20 years ago, when it was found to be equally sensitive to trace amounts of β-glucan (Kakinuma et al., 1981). This is thought to be because the horseshoe crab recognizes the occurrence of external bleeding by contact with such microbial substances, which are copious in the surroundings. At the same time they are able to defend themselves from invasion by fungi and gram-negative bacteria by clotting their blood in situ. The initial coagulation processes of these two
substances, however, differ. Endotoxin sets off a coagulation cascade by activating Factor C, while β-glucan activates Factor G (Figure 11.1). Thus, Factor G was used in coordination with the common downstream coagulation enzymes to develop a test kit for determining β-glucan in blood. Furthermore, patients with deep mycosis were shown, for the first time, to have high levels of β-glucan in their blood (Obayashi et al., 1985 and 1992). The method is extremely sensitive and is now widely accepted by clinicians in Japan for the care of immunocompromised patients, in whom the surveillance of opportunistic fungal infection is essential. Moreover, this method is especially helpful in corroborating the diagnosis of Pneumocystis Carinii pneumonia, in which plasma β-glucan is invariably high, to such an extent that its elevation is now deemed a diagnostic criterion by some experts.
