The organs of immunity include the central and peripheral organs of immunity. In mammals, the central immune organs include the thymus and bone marrow; in birds, the central immune organs include the thymus and bursa. Peripheral immune organs include lymph nodes and spleen. (Source: Laboratory instruction, Department of Immunology, School of Basic Medical Sciences, Peking University)
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Fetal thymus Thymus tissue section Human spleen tissue section Human lymph node section Chicken bursa Move I. Thymus Schematic diagram of thymus at different ages
The thymus is located in the upper part of the thoracic mediastinum, behind the sternum. The thymus grows rapidly in the embryonic period and within 2 years of age after birth, and its volume is large; it still develops rapidly after 2 years of age and during puberty; however, it begins to atrophy after puberty, and is gradually replaced by adipose tissue, which occupies an important position in the establishment of the immune function of the organism. Some of the lymphocytes in bone marrow migrate to the thymus, and under the influence of thymosin, they proliferate and differentiate into T-cells with immune function, which are then transported to lymph nodes, spleen and other peripheral immune organs to play their immune function through the bloodstream. If the thymus is removed during the neonatal period or if a tumor grows in the thymus, the cellular immune function will be significantly reduced, and the patient will often die due to infection.
Organizational structure of the thymus: the thymus is divided into two lobes, the surface is covered with a layer of connective tissue peritoneum, which penetrates deep into the parenchyma and divides the parenchyma into a number of lobules. The outer layer of thymus lobules is cortex and the inner layer is medulla. The thymic cortex is divided into a superficial cortical area and a deep cortical area. In the cortical zone, 85-90% of the cells are immature T cells. The medullary zone contains a large number of thymic epithelial cells and sparsely scattered mature thymocytes, monocytes-macrophages, and DCs.
Specimen: Fetal thymus, thymus tissue section
Figure 1. 
II. Bone marrow (Bone marrow)
Bone marrow is located in the marrow cavity and is divided into red bone marrow and yellow bone marrow. The red bone marrow is hematopoietic and consists of hematopoietic tissue and blood sinuses. Hematopoietic tissue consists mainly of stromal cells and hematopoietic cells. Stromal cells include reticulocytes, fibroblasts, vascular endothelial cells, macrophages, and so on. The bone marrow is the site where all types of blood cells and immune cells occur. The bone marrow is also the site of B cell differentiation and maturation and the site of humoral immune response. Therefore, the bone marrow is both a central and peripheral immune organ.
Specimen: Bone Marrow Tissue Section
III. Bursa of Fabricius
In the avian species, the bursa of Fabricius is a capsule located above the cloaca and filled with lymphoid tissue in which lymphocytes are constantly under the influence of cystin to develop into B lymphocytes. If the bursa is removed in the neonatal period, antibody formation is impaired, whereas cellular immunity is not affected. The avian bursa has a similar effect to mammalian bone marrow on B lymphocyte development.
Specimen: chicken bursa
IV. Lymphoid node
The parenchyma of the lymph node is divided into cortical and medullary regions. The cortical area is divided into superficial cortical area and deep cortical area. The superficial cortical area is close to the subepithelium, where B cells settle, and is called the thymus-independent area. In this area, large numbers of B cells gather to form lymphoid follicles, or lymphoid nodules. The deep cortical area between the superficial cortical area and the medulla is the paracortical area, where T cells settle, and is called the thymus-dependent area. The medullary area consists of medullary cords and medullary sinuses. The medullary cords are composed of densely packed lymphocytes, mainly B cells and plasma cells.
Specimen: Human lymph node section 
Figure 2. Lymph nodes
V. SpleenThe parenchyma of the spleen can be divided into white and red medulla. The white medulla is dense lymphoid tissue and is a T-cell area. The narrow area at the junction of the white and red medullas is the marginal zone, which contains T cells, B cells, and more Mφ. The red medulla consists of medullary cords and medullary sinusoids. The medullary cord is a cord-like tissue containing mainly B cells, plasma cells, Mф and DC.
Specimen: human spleen tissue section
Figure 3.
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