The experimental results indicate that the walls of the lower respiratory tract are relatively as impermeable for antibodies as they are for serum proteins. The kind of serum employed does not influence this fact. Antibodies contained in homologous (rabbit) serum failed to enter the blood stream when injected into the trachea in greater concentration than those contained in heterologous (guinea pig and cow) sera.

That a little serum and its contained substances are absorbed after intratracheal injection has been shown both by the experiments dealing with sensitization and anaphylaxis previously reported and by the experiments recorded in this paper.

Besredka's assertion that the laryngotracheal method is an efficient one for the administration of serum in the treatment of disease has not been borne out by the experimental evidence here presented. From the results one would conclude that it is one of the least efficient routes for administration.

In contrast to the relative impermeability for proteins of the mucous membranes of the lower respiratory tract is the relative permeability of the endothelium of the peritoneal cavity. Antibody is there absorbed readily, whether it be in heterologous or in homologous serum. The indications are that a considerable portion of the antibody injected into the peritoneal cavity reaches the blood stream in 2 or 3 hours.

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