Presence of Antigen-Binding Cells for Five Diverse Antigens at the Onset of Lymphoid Development: Lack of Evidence for Somatic Diversification during Ontogeny

Cells capable of binding antigen can be detected by half term in mouse, rabbit, and chick embryos, preceding the appearance of most organized lymphoid tissue, some other surface markers, and most immune function (help, antibody synthesis). Binding cells for five diverse antigens are all present at this early stage of development and in approximately the same order of magnitude, depending on the size of the antigen. The number of antigen-binding cells increases during ontogeny, although their frequency in the thymus decreases from the time the organ can first be assayed to the time of birth. Antigen binding occurs via specific, Ig-like receptors. A proportion of cells is able to bind two antigens noncompetitively when both are added to the cell suspension. The ability of the lymphoid system to recognize a diverse array of epitopes early in development suggests that genes coding for most antibody specificities are present in the germ-line complement.