Mixed lymphocyte reactions occur when mouse spleen cell populations depleted of thymus-derived (T) lymphocytes are cultured with allogeneic target cells inactivated by mitomycin C or X irradiation, and when F1 hybrid responder cells are cultured with inactivated parental target cells. These responses might be interpreted as indicating that T lymphocytes are not required for responsiveness and that F1 lymphocytes recognize parental alloantigens. Data reported here indicate that the more likely explanation for these surprising results is that inactivated target cells recognize the "responding" cells and this recognition leads to the response observed.

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