Lymphoid organs are primary and secondary. Primary lymphoid organs are those in which immature lymphocytes become antigen sensing cells. These organs are bone marrow and thymus. Secondary lymphoid organs are those that receive cells from bone marrow and thymus. These organs are lymph nodes, spleen, peyer's patches etc. Spleen is known as the blood bank of the body. Histologically it shows two regions: the red pulp and the white pulp. The red pulp contains damaged erythrocytes as spleen is the graveyard for these cells. White pulp contains lymphocytes and phagocytes and these cells trap the microorganisms.