Against the concept that starch is inert immunologically and, therefore, an ideal substrate for in vitro studies on leucocytes, the results here recorded demonstrate that there are at least two components of serum which are specifically required for phagocytosis of starch. The available evidence suggests that the heat-labile component is C′, and the heat-stable component is specific Ab. The latter, which is a serum globulin, conforms to the definition of Ab from three standpoints: (1) it is adsorbed to the starch granule at low temperature and cannot be eluted by washing with saline at pH 7–0; (2) although it is found in ‘normal’ serum, it appears in increased quantities in the serum of animals injected with starch; and (3) once combined with the starch granule, it exerts three in vitro immunological effects, i.e. opsonization, induction of immuneadherence, and agglutination. As evidence that the latter reactions are due to specific Ab, the enhancement of phagocytosis and the induction of immune-adherence are manifested only in the presence of C′.

The measurement of Ab or C′ has been complicated by the finding that fresh normal serum to be employed as C′ has invariably contained Ab to starch. This situation is similar to that which exists with a variety of micro-organisms, e.g. Micrococcus aureus, against which the majority of normal animals and human beings possess circulating Ab. To obtain C′ free of Ab, fresh guinea-pig serum was adsorbed in the cold with starch. Despite the low temperature which would be expected to reduce the C′ fixation, about 30–40% of C′ activity was lost during adsorption for 60 min. at 0° C. When the adsorption was performed in the absence of Mg2+ and Ca2+, the loss of C′ was lessened considerably. This is interpreted to mean that at least two factors contributed to the loss of C′ cited above: (1) the specific fixation of some or all components of C′ to the starch-Ab complex, which requires the presence of Mg2+ and Ca2+, and which occurs even in the cold; and (2) the non-specific action of starch in adsorbing C′, or perhaps destroying C′, which does not require Mg2+ or Ca2+.