During ontogeny, Ia-bearing accessory cells are found early in the thymus but late in the spleen.

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Abstract
The ontogeny of Ia-bearing accessory cells was studied in mice. Ia-bearing adherent cells from the thymus, consisting predominantly of macrophages, were found from birth. These adherent cells were able to present antigen, as measured by their ability to induce immune T-cell proliferation. In contrast, Ia-bearing adherent cells from the spleen were not found until the second week of life, and their antigen-presentation function was not present until later. The differential ontogeny of Ia-bearing accessory cells at these sites may be important in both development of immune competence and the restriction of autoimmunity.