Main stages of lymphopoiesis
Lymphocyte development is divided into two phases:
- Antigen-independent differentiation. Takes place in the central hematopoietic organs (red bone marrow and thymus). Cells mature and form their receptor repertoire prior to contact with any actual antigen. In adults, this process slows down.
- Antigen-dependent differentiation. Occurs in peripheral lymphoid tissue (lymph nodes, spleen) only after a lymphocyte encounters its specific antigen. The cell is activated, divides, and forms a clone, which essentially constitutes the immune response.
Developmental lineages and migration
From the common lymphoid progenitor (class II), unipotent progenitors (class III) are formed, giving rise to two main pathways:
- B lymphocytes. Complete their entire early antigen-independent development cycle in the red bone marrow. Their final stage (following antigen encounter) is the plasma cell, which produces antibodies.
- T lymphocytes. Their progenitors are formed in the bone marrow, but then migrate to the thymus, where they undergo complex antigen-independent maturation and selection.
There is also a separate lineage—natural killer (NK) cells. They are formed in the bone marrow, but their development is not accompanied by genetic rearrangement, so they lack unique antigen receptors.
Genetic rearrangement (recombination)
The human genome cannot accommodate pre-existing genes for billions of possible antibodies. Therefore, progenitor DNA contains only sets of short gene segments. During the antigen-independent stage, special enzymes excise one random segment at a time and "splice" them together into a unique receptor gene.
This ensures combinatorial diversity—each cell receives its own unique receptor (BCR in B cells and TCR in T cells). The rule of allelic exclusion applies: if rearrangement on one chromosome is successful, the second one is blocked. This guarantees that the lymphocyte will react to only one type of antigen (monospecificity).
Lymphocyte selection
The generation of unique receptors frequently leads to errors or the emergence of cells dangerous to the body's own tissues. To prevent this, lymphocytes undergo rigorous selection:
- Positive selection. Receptor functionality is tested. In the thymus, T cells must successfully bind to major histocompatibility complex (MHC) molecules on epithelial cells. Those that fail (possessing a defective receptor) die via apoptosis. At this stage, T cells lose one of their coreceptors, becoming future killers (CD8+) or helpers (CD4+).
- Negative selection. Tests for auto-reactivity. Cells that survived the first round interact with the body's own self-antigens. If a lymphocyte binds to them too strongly, it is destroyed.
As a result, only "naive" lymphocytes capable of recognizing foreign agents survive. They leave the central organs and populate the T- and B-zones of peripheral lymphoid organs.
Briefly: answers to topical questions
What specific stages do B lymphocytes undergo during antigen-independent differentiation in the red bone marrow?
During antigen-independent differentiation in the red bone marrow, B lymphocytes go through several sequential stages of development.
- Lymphopoietic progenitor cell — Class II, common to both B and T lineages.
- Pro-B lymphocyte — Class III (unipotent progenitors).
- Pre-B lymphocyte (B lymphoblast) — Class IV (blasts), where active cell proliferation takes place.
- Immature B lymphocyte — Class V (maturing cells).
- Mature B lymphocyte — Class VI.
Which cells are formed at the final stage of antigen-dependent B lymphocyte differentiation?
At the final stage of B cell differentiation, a plasma cell is formed.
- Plasma cell — the terminal stage of B cell differentiation, an antibody producer.
In the germinal centers of peripheral organs during antigen-dependent B lymphocyte differentiation, the following are also described:
- centroblasts — B cells at the stage of somatic mutagenesis and proliferation;
- centrocytes — B cells at the selection stage;
- B immunoblasts — cells at the stage of division and antibody class switching.
To which specific morphological zones of peripheral organs do T and B lymphocytes migrate after maturation?
Following maturation, lymphocytes reside in specialized zones of peripheral lymphoid organs.
- T-lymphocytes — in thymus-dependent zones: paracortical zone / paracortex of lymph nodes; in the spleen — periarterial lymphatic sheath, PALS of the white pulp.
- B lymphocytes — in B cell zones: lymphoid follicles; antigen-dependent proliferation and differentiation of B cells occur in the germinal centers of the follicles.
Class and exam questions
What is the difference between antigen-independent and antigen-dependent stages?
The antigen-independent stage takes place in the bone marrow and thymus; its goal is to generate a cell with a unique receptor prior to antigen encounter. The antigen-dependent stage occurs in the periphery—this is the clonal expansion of the cell after recognizing a specific enemy.
Why do most T lymphocytes die in the thymus?
They are destroyed during selection due to defective receptors (failing positive selection) or because their receptors attack the body's own proteins (failing negative selection).
How does a T lymphocyte recognize an antigen?
A T cell cannot see free antigens. It recognizes only an antigen fragment presented to it by another cell in a complex with an MHC (major histocompatibility complex) protein.
What else to study on this topic
- Classes of hematopoiesis (from II to VI) in lymphopoiesis
- Mechanism of MHC restriction and functions of class I and II MHC molecules
- Development and functions of NK cells
- Morphological characteristics of lymphoblasts and immature lymphocytes
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