Protochordate immunity. I. Primary immune response of the tunicate Ciona intestinalis to vertebrate erythrocytes

Serum from the tunicate Ciona intestinalis contains a low titer, natural agglutinin for a variety of vertebrate erythrocytes. Primary injections of 8.5 × 10³ human erythrocytes or 7.0 × 10² duck erythrocytes into the tunic tissues or perivisceral cavity were agglutinated and then cleared by phagocytosis within 24 hr. Higher concentrations of human or duck erythrocytes injected into the tunic tissues were encapsulated. Concomitant with clearance or encapsulation, serum agglutinin levels decreased and then returned to normal within 24 hr. Tunicates are classified taxonomically with the vertebrates in the phylum Chordata. Since phagocytosis and encapsulation reactions are typical of most invertebrates, the results of this study suggest that tunicate internal defense mechanisms are more closely allied to invertebrates than to vertebrates.