Primary lymphoid organs are also known as central lymphoid organs. These are the organs where differentiation, proliferation and maturation of pleuripotent stem cells into immuno competent cells take place.
Thymus and avian bursa of Fabricus or bursa equivalent tissue in the bone marrow are categorized as primary lymphoid organs.
Thymus helps in the differentiation and maturation of hemopoitic stem cells in to T cells. Bursa of fabricus or its equal region in bone marrow is responsible for the differentiation and maturation of B cells. Even though T and B cells in primary lymphoid organs are competent, they don’t show any immune response.
Since interaction with antigens is necessary for immune response, in the absence of antigens immune cells in primary lymphoid organs remain inactive.