Primary lymphoid organs
The primary organs of the immune system include the bone marrow and the thymus. The bone marrow acts not only as an organ of hematopoiesis but also as a crucial center of immunogenesis, where the events of the humoral response take place.
Contrary to outdated views, in adult mammals, early precursors and stem cells are distributed uniformly throughout the bone marrow (in the form of islands) rather than being concentrated at the periphery of the bone canal. Cell proliferation and maturation occur in meshes formed by reticular cells. Gradually, maturing elements shift toward the center, enter the sinusoids, and pass into the bloodstream.
Of all lymphoid cells in the bone marrow, about 60% are at the maturation stage, and 40% are mature forms. The B-cell lineage is characterized by extremely high productivity. Among the B-lineage cells, the following are distinguished:
- Pre-B cells (with immunoglobulin $\mu$-chain) — 5.8%;
- Mature B cells (with surface IgM) — 2.5%;
- Memory B cells (expressing IgG or IgA) — 1.3%;
- Plasma cells (antibody producers) — 1–2%.
Spleen
The spleen is a large secondary lymphoid organ, whose tissue is functionally divided into several zones:
- White pulp. Structurally resembling lymph nodes, it contains T, B, and NK cells. Antigens enter here via the hematogenous route (through the bloodstream). NKT and NK cells play an important role here.
- Red pulp. Erythroid and myeloid elements predominate here, while immunocytes include blasts and plasma cells. This zone maintains blood cell homeostasis and forms a microenvironment for antibody-forming and memory cells.
- Marginal zone. An intermediate region separating the white and red pulp. A specific subpopulation of MZB cells is localized here.
Unencapsulated lymphoid aggregates
This group of structures provides local (mucosal) immunity in areas with a high microbial load:
- Tonsils — aggregates of follicles in the digestive and respiratory tracts. They are dominated by IgA+ B lymphocytes surrounded by myeloid and T cells.
- Peyer's patches are structures covered by follicular epithelium. It contains specialized M cells through which antigens enter. Beneath the epithelium (in the dome) are macrophages, dendritic cells, and memory lymphocytes.
- Solitary follicles — analogous to primary follicles, serving as sites for the initiation of the immune response.
- Cryptopatches — contain lymphoid tissue inducer cells (LTICs). They are responsible for the formation of new lymphoid zones in the postnatal period.
- Appendix — a conglomerate of follicles, part of mucosal immunity (the specific purpose of the organ remains unknown so far).
Diffuse lymphoid tissue
Diffuse tissue includes lymphocytes scattered throughout the mucous membranes, skin, and non-lymphoid organs (liver, lungs).
In the mucous membranes and skin, immunocytes are distributed directly between epithelial cells. Various subpopulations of memory T cells (including $\gamma\delta$ T cells) predominate there. Their main task is to form the body's first line of defense and initiate a secondary immune response.
In non-lymphoid organs (e.g., the liver), lymphocytes often have an unusual phenotype. For instance, the liver is dominated by NK and NKT cells, while T and B lymphocytes are predominantly represented by memory cells.
Briefly: answers to topical questions
Which organs are part of the secondary (peripheral) lymphoid organs?
Secondary (peripheral) lymphoid organs include encapsulated and unencapsulated structures. These are:
- Lymph nodes — monitor specific areas of the body from which lymph flows to them.
- Spleen — monitored via hematogenous pathways of foreign agent distribution.
- Peyer's patches — associated with the intestine, containing structures for antigen transport across the epithelial barrier.
- Lymphoid structures of barrier tissues — associated with mucous membranes and skin, forming zones of entry for the bulk of pathogens.
What is the histological structure and main functions of the thymus?
The thymus is a central lymphoid organ whose main function is to ensure the development of T lymphocytes. Externally, the organ is covered by a connective tissue capsule from which trabeculae extend inward. The thymus tissue is divided into the following zones:
- Cortex — contains cortical epithelial cells and densely packed thymocytes.
- Medulla — contains medullar epithelial cells, dendritic cells, macrophages, and Hassall's corpuscles.
- Corticomedullary junction — the boundary region between the cortex and medulla.
The connective tissue stroma forms the capsule, interlobular septa, and perivascular space.
What structural zones are distinguished in a lymph node?
Several structural zones are distinguished in a lymph node, ensuring lymphocyte distribution and lymph flow. The main zones include:
- Cortical zone — contains lymphoid follicles, which serve as the B zone.
- Paracortical zone — located deeper than the cortex, representing the thymus-dependent T zone.
- Mixed zone — the space between the follicles and the paracortical zone with a mixed population of T and B cells.
- Medulla — contains medullary cords (aggregates of macrophages and plasmacytes) and medullary sinuses.
The lymphatic sinus system is also distinguished: marginal (subcapsular), perinodular (trabecular), and medullary sinuses.
What is the structure and function of the palatine tonsils?
The palatine tonsils (tonsillae palatinae) are clusters of lymphoid tissue that perform a protective function and provide the initial events of the primary and secondary immune response.
- Epithelium — non-keratinized stratified squamous, forming 10–20 deep branching crypts and infiltrated by immune cells.
- Lamina propria — contains lymphatic follicles (B zone, rich in B1 lymphocytes) and interfollicular areas (T zone).
- Capsule — formed by dense fibrous connective tissue, from which septa extend.
The tonsils are located between the palatine arches and are part of the pharyngeal lymphoid ring.
Class and exam questions
How are stem cells distributed in the bone marrow of an adult human?
Contrary to old concepts of peripheral localization, modern data show that early precursors and hematopoietic stem cells are distributed evenly throughout the bone marrow in the form of islands.
By what pathway do antigens enter the white pulp of the spleen?
Unlike classical lymph nodes, where antigens are brought with lymph, antigens are delivered to the white pulp of the spleen exclusively via the hematogenous route (through the bloodstream).
What are cryptopatches and what is their function?
Cryptopatches are unencapsulated aggregates dominated by lymphoid tissue inducer cells (LTICs). They serve as a source of these cells and ensure the formation of new lymphoid zones in mucous membranes after birth.
Through which cells do antigens enter Peyer's patches?
Antigen enters Peyer's patches via specialized M cells located within the follicular epithelium that separates the lymphoid tissue from the organ lumen.
What else to study on this topic
- Structural features and immunological role of the thymus as a primary lymphoid organ
- Mechanism of M cell action and antigen transport into the dome of Peyer's patches
- The B-cell maturation process: from pre-B cells to plasma cells
- The role of splenic marginal zone B cells (MZBs) in the immune response
- Phenotypic diversity of lymphocytes in the liver and lungs
Master the entire topic in the "Immunology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Organs of the Immune System" and 723 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 23 video lessons on the subject
Other topics in the "Adaptive Immunity" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.