Veterinary Immunology and Serology

ORGANS OF THE IMMUNE SYSTEM

• Lymphoid organs are the important organs of the immune system.
• Lymphocytes are the highly dynamic cells and large number of them recirculating as individual cells in blood, lymph and tissue fluids.
• Lymphocytes are the predominant cells in lymphoid organs. They can recognize, respond and eliminate antigens.
• Lymphocytes either produce humoral or cell mediated immune (CMI) response or both. Lymphoid organs can be classified into two groups
  • Primary lymphoid organs
  • Secondary lymphoid organs

Primary lymphoid organs 

• The organs that regulate the production and differentiation of lymphocytes into immunocompetent lymphocytes without antigenic stimulation are called primary lymphoid organs.
• Primary lymphoid organs are also called generative organs or central lymphoid organs.
• In primary lymphoid organ, lymphocytes first express antigenic receptors and mature both phenotypically and functionally without any antigenic stimulation.
• Mature lymphocytes fall into two major populations, T cells and B cells depending on at which primary organ they were matured.
• T cells mature in thymus where as B cells in Bursa of Fabricius (in birds) or bone marrow (primates and rodents) or Peyer’s patches (in ruminants and pigs).
• Examples of primary lymphoid organs: bone marrow, thymus, Bursa of Fabricius, some Peyer’s patches and fetal liver.

Secondary lymphoid organs

• They are also called peripheral lymphoid organs.
• In these organs antigens are trapped and immune response is generated based on foreign antigenic stimulation.
• These organs are rich in macrophages and dendritic cells (which trap and process antigen) and also T and B cells (which mediate immune response).
• Examples of secondary lymphoid organs: Tonsil, spleen, lymph nodes, mucosal associated lymphoid tissues (MALT), gut associated lymphoid tissues (GALT).