Introduction to the Secondary Organs of the Immune System

The secondary lymphatic organs represent the location where the defense battles take place. In them the T- and B-lymphocytes that have matured in the thymus and in the bone marrow develop further when they come into contact with antigens, leading to a clonal proliferation. Specific proliferation zones form for the two lymphocyte groups. Thereby effector and regulator cells. The anlage material for the secondary lymphatic organs has amesenchymal origin. It forms in connection with the differentiation of the lymph- and vascular systems.[1]

Various types of organized lymphoid tissues are located along the vessels of the lymphatic system. Some lymphoid tissue in the lung and lamina propria of the intestinal wall consists of diffuse collections of lymphocytes and macrophages.

Other lymphoid tissue is organized into structurescalled lymphoid follicles, which consist of aggregates of lymphoid and nonlymphoid cells surrounded by a network of draining lymphatic capillaries. Until it is activated by antigen, a lymphoid follicle—called a primary follicle—comprises a network of follicular dendritic cells and small resting B cells. After an antigenic challenge, a primary follicle becomes a larger secondary follicle—a ring of concentrically packed B lymphocytes surrounding a center (the germinal center) in which one finds a focus of proliferating B lymphocytes and an area that contains nondividing B cells, and some helper T cells interspersed with macrophages and follicular dendritic cells.[2]

Lymph nodes and the spleen are the most highly organized of the secondary lymphoid organs; they comprise not only lymphoid follicles, but additional distinct regions of  T cell and B-cell activity, and they are surrounded by a fibrous capsule. Less-organized lymphoid tissue, collectively called mucosal-associated lymphoid tissue (MALT), is found in various body sites. MALT includes Peyer’s patches (in the small intestine), the tonsils, and the appendix, as well as numerous lymphoid follicles within the lamina propria of the intestines and in the mucous membranes lining the upper airways, bronchi, and genital tract.[2]

References:

[1]  17.2 Lymphatic tissue, Human Embryology, Organogenesis from www.embryology.ch

[2] Kuby, 5^th edition, Unit 2: Cells and organs of immune system, pg 46-47, Immunology.