The relative roles of platelets and macrophages in clearing particles from the blood; the value of carbon clearance as a measure of reticuloendothelial phagocytosis

The correlation between detailed kinetic studies and ultrastructural localisation of particles during carbon clearance in normal animals illustrates that single order kinetics are not always obtained and phagocytosis is not always the major clearing process. During the early stages of clearance adherence of particles to platelets and to macrophage surfaces without immediate phagocytosis, as well as aggregation of particles within the blood are important controlling factors in the usual light absorbance techniques used to measure rates of clearance. The importance of the various processes involved varies depending upon the dose of carbon used and the time for which clearance is followed. The ultrastructural studies suggest an early constant rate of phagocytosis by Kupffer cells despite wide changes in dose of particles. At doses of 1 or 2 mg/100 g body weight of colloidal carbon the ultrastructural evidence shows that phagocytosis is the major early clearing process while at doses of 8 mg/100 g body weight and above the other processes are more important in the early clearance. Most clearance studies in the literature do not provide sufficient information to be certain that phagocytosis is the major process involved and such information can only be provided by detailed kinetic studies correlated with ultrastructural localisation of the particles. Many previous studies have used doses of colloidal carbon which invoke a major degree of the clearance processes other than phagocytosis and such studies are often based on a limited number of samples taken over a period when phagocytosis is not the only rate-controlling factor. Particle clearance studies should at least provide information concerning the possible role of platelets before the rates are considered to be measures of phagocytic function.