When cells from lymph nodes or thoracic duct of mice hyperimmunized with protein antigens are cultivated on embryo monolayers in the presence of the antigen, numerous clones of mast cells appear. The histochemical and ultrastructural characteristics of the cells permit their identification as mast cells and distinguish them from the phagocytic histiocytes that usually arise in abundance in similar cultures from unimmunized mouse cells or from immunized mouse cells cultured in the absence of the antigen. Only a few colonies of mast cells appeared in the latter cultures. The basis for the induction of mast cell differentiation is not known.

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