Primary lymphoid organs Bursa of Fabricius of birds, bone marrow and thymus gland of mammals constitute the primary lymphoid organs involved in the production and early selection of lymphocytes. These lymphocytes become dedicated to a particular antigenic specificity . Only when the lymphocytes mature in the primary lymphoidal organs, they become immunocompetent cells . In mammals, B cell maturation occurs in the bone marrow and T cells maturation occurs in the thymus.
Thymus The thymus is a flat and bilobed organ located behind the sternun, above the heart. Each lobe of the thymus contains numerous lobules, separated from each other by connective tissue called septa. Each lobule is differentiated into two compartments, the outer compartment or outer cortex, is densely packed with immature T cells called thymocytes, whereas the inner compartment or medulla is sparsely populated with mature thymocytes. One of its main secretions is the hormone thymosin .
It stimulates the T cell to become mature and immunocompetent . By the early teens, the thymus begins to atrophy and is replaced by adipose tissue (Fig. . ).
Thus thymus is most active during the neonatal and pre-adolescent periods . Fig. . Primary lymphoid organ - Thymus A) Location B) Structure 7K\URLG JODQG 7UDFKHD 6HSWDH /REXOH /XQJV +HDUW 7K\PXV $ % 5LJKWOREH /HIWOREH Based on their functions, they are classified into primary or central lymphoid organs and secondary or peripheral lymphoid organs .
The primary lymphoid organs provide appropriate environment for lymphocytic maturation. The secondary lymphoid organs trap antigens and make it available for mature lymphocytes, which can effectively fight against these antigens. Bursa of Fabricius is a primary lymphoid organ of birds. It is attached to the dorsal side of the cloaca.
B lymphocytes mature in the bursa and bring about humoral immunity. Fig. . Lymphoid organs in human body XII Std Zoology Chapter XII Std Zoology Chapter Immunology Lymph node Lymph node is a small bean-shaped structure and is part of the body’s immune system.
It is the first one to encounter the antigen that enters the tissue spaces. Lymph nodes filter and trap substances that travel through the lymphatic fluid. They are packed tightly with white blood cells, namely lymphocytes and macrophages. There are hundreds of lymph nodes found throughout the body.
They are connected to one another by lymph vessels. Lymph is a clear, transparent, colourless, mobile and extracellular fluid connective tissue. As the lymph percolates through the lymph node, the particulate antigen brought in by the lymph will be trapped by the phagocytic cells, follicular and interdigitating dendritic cells. Mantle zone Sinuses Medullary cords Efferent lymphatic vessel Paracortex Cortex Follicle Afferent lymphatic vessel Fig.
. Secondary lymphoid organ – Structure of lymph node Bone marrow Bone marrow is a lymphoid tissue found within the spongy portion of the bone. Bone marrow contains stem cells known as haematopoietic cells. These cells have the potential to multiply through cell division and either remain as stem cells or differentiate and mature into different kinds of blood cells.