Appearance of mast cells in bone marrow, peripheral blood and spleen of immunized rats.

The appearance of mast cells and their progenitors was studied in rats. The animals were immunized according to a protocol utilizing 2-week periods of daily administration of small doses of ovalbumin either as aerosol or as subcutaneous injection without adjuvant. These procedures are known to induce allergic responses of the IgE-mediated type. The immunization procedures resulted in increased numbers of mature mast cells in bone marrow. There were also increased numbers of progenitors, as indicated by increased maturation of mast cells in cultures of bone marrow cells from immunized animals compared with nonimmunized controls. Bone marrow cells in cultures from immunized and nonimmunized animals showed similarly increased maturation of mast cell progenitors when stimulated with conditioned medium from mixed lymphocyte reaction cultures. The number of progenitors was considerably lower in peripheral blood and spleen than in bone marrow. However, cultures of cells from peripheral blood and spleen of aerosol-immunized rats contained cells which matured into mast cells after addition of medium from mixed lymphocyte reaction cultures. The study shows that in immune responses similar to those seen in allergic individuals mast cells are normal constituents.