Lymphoid System

The lymphoid system is composed of the organs that produce lymphocytes (bone marrow and thymus), and organs largely composed of lymphoid tissue (lymph nodes, spleen, and the tonsils of the pharyngeal lymphoid ring). Mature lymphocytes are naive when they leave the primary lymphoid organs (bone marrow and thymus). Immunological reactions are initiated by the presentation of antigens to lymphocytes by antigen-presenting cells within secondary lymphoid organs or mucosa-associated lymphoid tissues.

Other Names

Synonyms

• Lymphoid organs

Related terms

• Lymphatic system
• Hematopoietic tissues
• Lymphoid tissues
• Immune system

Description

The lymphoid system is divided into primary and secondary lymphoid organs. Although, not formally part of the lymphoid system because they are tissues in other organs, mucosa-associated lymphoid tissues are included in this listing because these tissues are functionally similar to the tonsils.

Primary lymphoid organs

The primary lymphoid organs produce mature, naive, immuno-competent lymphocytes.

• Bone marrow
• Thymus

Secondary lymphoid organs

The secondary lymphoid organs are organs in which lymphocytes respond to antigen presentation. The parenchyma of these organs is composed of lymphoid tissue.

• Dense (encapsulated) lymphoid organs
  • Spleen
  • Lymph Nodes
• Pharyngeal lymphoid ring
  • Lingual tonsils
  • Palatine tonsils
  • Pharyngeal tonsil
  • Tubal tonsils

Mucosa-associated lymphoid tissue

Lymphoid tissue is present in the digestive, respiratory and urinary tracts, mainly within the lamina propria of the mucosa . Mucosa-associated lymphoid tissue (MALT) may take the form of diffuse infiltration by lymphoid cells (lymphocytes and plasma cells), lymphoid nodules, or aggregations of lymphoid follicles (e.g., Peyer's patches).

• Gut-associated lymphoid tissue (GALT)
• Bronchus-associated lymphoid tissue (BALT)
• Diffuse lymphoid tissue of urinary tract

History

How Lymphoid System Was Named

The lymphoid system is relatively new to anatomical nomenclature. The organs grouped as the lymphoid system in Terminologia Anatomica (1998), were listed in other systems in earlier nomenclatures (e.g., Nomina Anatomica). The term lymphatic system was dropped; lymphatic vessels remained within the cardiovascular system, and the lymph nodes and spleen were joined with the bone marrow, thymus, and pharyngeal lymphoid ring to form the lymphoid system.