Lymphoidei

Lymphoidei is a term used to describe the tissues, cells, and organs that comprise the lymphatic immune system. They are characterized by dense populations of lymphocytes within a reticular connective tissue framework and are organized to support the generation, maturation, and activation of immune responses.

The lymphoid system includes primary and secondary lymphoid organs. Primary lymphoid organs are sites of lymphocyte development and maturation, notably the bone marrow and the thymus. Secondary lymphoid organs and tissues include lymph nodes, the spleen, tonsils, and mucosa-associated lymphoid tissue (MALT) such as Peyer's patches; these structures concentrate antigens and provide environments for lymphocyte activation and clonal expansion.

Lymphoid tissue exists as organized organs and as diffuse lymphoid tissue. Lymphoid nodules and follicles often contain B cell zones with germinal centers, while T cells are commonly located in adjacent paracortical or periarterial regions. In secondary organs, antigen-presenting cells such as dendritic cells and macrophages interact with lymphocytes to initiate adaptive immune responses.

Cellular composition typically includes B lymphocytes, T lymphocytes, and natural killer cells, supported by antigen-presenting cells and reticular stromal elements. Functions of lymphoidei encompass surveillance of bodily tissues for pathogens, antigen processing and presentation, lymphocyte activation, and production of antibodies or cytotoxic responses.

Developmentally, lymphoid cells arise from hematopoietic stem cells. B cells mature in the bone marrow, while T cells mature in the thymus before migrating to peripheral lymphoid tissues. Clinically, lymphoid tissues are central to immunodeficiencies, lymphoid neoplasms such as lymphomas, and inflammatory disorders, making them essential for diagnosis and therapy.