The avian primary lymphoid organs, the bursa of Fabricius and the thymus, are crucial to the normal development of B and T lymphocytes in birds. Birds use gene conversion to produce different classes of immunoglobulins and this process occurs in the bursa of the Fabricius. The microenvironment of the bursa selectively expands those B-cell precursors that have undergone productive V(D)J recombination. On the other hand, the thymus constitutes the microenvironment for T lymphocyte differentiation and the acquisition of self-tolerance. Production of T cells in the thymus is controlled by a combination of positive and negative selection. The differentiation of T cells proceeds along two pathways characterized by the expression of αβ or γδTCRs. Immunologically mature lymphocytes enter the circulation and colonize the peripheral lymphoid organs.
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