Classification
Anomatically, the human immune system is divided into two major groups of organs. Their key difference lies in whether these processes depend on contact with foreign agents (antigens).
- Central organs: bone marrow and thymus. This is where immune cells are born and undergo initial, antigen-independent maturation.
- Peripheral organs: lymph nodes, spleen, Peyer's patches, as well as skin-associated lymphoid tissue (SALT) and mucosa-associated lymphoid tissue (MALT). Naive (untrained) lymphocytes migrate here. These organs are the sites of antigen-dependent processes: final maturation, antigen presentation, and the execution of the immune response.
Central organs
Bone marrow consists of stroma and hematopoietic tissue. It is a reservoir of undifferentiated stem cells where hematopoiesis (myelopoiesis and lymphopoiesis) takes place. T and B lymphocytes, as well as natural killer cells, are formed here. B lymphocytes leave for peripheral organs and, after encountering an antigen, can return as plasma cells to synthesize antibodies.
Thymus is located behind the manubrium of the sternum. It is a bilobed organ responsible for the maturation and selection of T lymphocytes. A thymic lobule consists of cortex and medulla, with the blood-thymus barrier located in the cortex. With age (after 30 years), the organ undergoes involution—tissue is replaced by adipose and connective tissue.
Peripheral organs and tissues
Lymph nodes are oval structures along the course of lymphatic and blood vessels. They consist of cortical (B-zone), paracortical (T-zone), and medullary zones. Upon contact with an antigen in the primary follicle, B lymphocytes are activated, forming a germinal center (secondary follicle) where cells divide, differentiate into plasma cells, and undergo immunoglobulin class switching.
Peyer's patches are lymphoid tissue in the intestinal mucosa. Their epithelium contains M cells that capture antigens from the lumen and deliver them to macrophages and dendritic cells. This is critical for the development of a proper immune response.
Spleen is a parenchymatous abdominal organ. In the fetus, it is responsible for hematopoiesis, but after birth, it becomes a crucial component of peripheral immunity (mainly humoral) and a blood cell reservoir. Its white pulp contains a T-zone (around arterioles) and a B-zone (follicles).
Briefly: answers to topical questions
Which cells are formed in the bone marrow during hematopoiesis?
During hematopoiesis, myeloid and lymphoid lineage cells are formed in the bone marrow.
- Myeloid lineage cells include monocytes, macrophages, neutrophils, eosinophils, basophils, mast cells, dendritic cells, erythrocytes, and megakaryocytes.
- Lymphoid lineage cells include T lymphocyte subpopulations (CD8+, CD4+), B lymphocytes (B1, B2), and NK cells (natural killer cells).
What structures make up the blood-thymus barrier?
The blood-thymus barrier is formed by blood vessels and stromal cells of the thymic cortex. It includes:
- Blood capillaries and arterioles — isolate maturing lymphocytes from blood antigens.
- Supporting reticuloepithelial cells — flat cells that surround vessels and form the framework of the cortex.
What functions does the spleen perform in an adult?
In adults, the spleen (lien, splen) performs a number of vital functions, acting as a central organ of peripheral immunity.
- Immune — providing an adaptive response, antigen-dependent lymphocyte proliferation, antibody production, and antigen phagocytosis.
- Storage — a reservoir for blood (up to 300 ml), erythrocytes, granulocytes, and platelets (up to 30% of the pool).
- Filtration — removal of aged erythrocytes, platelets, and microorganisms.
- Hematopoietic — execution of lymphopoiesis.
- Metabolic — participation in iron metabolism via recycling from hemoglobin of destroyed erythrocytes.
What anatomical structures are part of MALT?
Mucosa-associated lymphoid tissue (MALT) includes structures of the gastrointestinal, respiratory, and genitourinary tracts.
- Waldeyer's tonsillar ring — includes six tonsils (lingual, pharyngeal, two palatine, and two tubular).
- Intestinal lymphoid tissue — solitary follicles and Peyer's patches in the small intestine wall, as well as lymphoid aggregations in the appendix wall.
- Bronchus-associated lymphoid tissue (BALT) — lymphoid nodules in the walls of the airways.
MALT also includes diffuse lymphoid tissue represented by scattered lymphocytes outside follicles.
Class and exam questions
What is the fundamental difference between central and peripheral organs of the immune system?
In central organs, cell differentiation occurs independently of the antigen. In peripheral organs, final maturation and initiation of the immune response occur only after encountering a foreign agent.
Where do T lymphocytes and B lymphocytes mature?
T lymphocytes mature in the thymus (hence the letter "T"). B lymphocytes are formed and undergo primary development in the bone marrow.
What is a germinal center?
This is the center of proliferation inside a lymph node follicle or Peyer's patch, formed in response to an antigen. Active division of B lymphocytes takes place there.
What else to study on this topic
- Stages and mechanisms of hematopoiesis in the bone marrow
- The blood-thymus barrier and the structure of a thymic lobule
- The mechanism of immunoglobulin class switching in a lymph node
- The role of M cells in the pathogenesis of allergic reactions
- Structure of the red and white pulp of the spleen
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