Main stages of development
The process of humoral immunity formation is divided into two major phases, which include several consecutive stages:
- Inductive phase. Includes antigen recognition and primary B cell activation, as well as their subsequent proliferation (clonal expansion — multiplication of stimulated lymphocytes).
- Productive phase. Covers the stage of B cell differentiation into two pools: effector plasma cells (synthesizing antibodies) and long-lived memory B cells. The core content of this phase is the direct production of immunoglobulins.
How a B lymphocyte recognizes an antigen
Unlike T cells, the antigen-binding receptor of a B lymphocyte (BCR) is capable of interacting with an antigen in its native (uncleaved) form.
For the successful triggering of intracellular cascades, cross-linking (aggregation) of receptors on the membrane is required. Transmission of the first activation signal proceeds in two directions:
- Via the receptor complex: conformational changes in the membrane immunoglobulin are transmitted to the Igα/Igβ dimer (CD79a/b), activating receptor kinases.
- Via the complement system: the antigen-antibody complex activates the classical complement pathway. The C3d fragment binds to the CD21 coreceptor (part of the CD19 complex).
The joint action of tyrosine kinases from CD79 and CD19 forms the basic first signal. Inside the cell, kinases of the Src family and Syk kinase are engaged, ultimately leading to the activation of crucial transcription factors: NFAT, AP-1, and NF-κB.
Second signal: interaction with T helper cells
Antigen binding alone is not sufficient for full differentiation. The B lymphocyte acts as a highly specific antigen-presenting cell (APC): it engulfs the recognized antigen, cleaves it, and displays peptide fragments in complex with MHC-II on its surface.
In the interfollicular space, this complex is recognized by the T-cell receptor (TCR) of a specific type 2 T helper cell (Th2). An immunological synapse is formed, where an exchange of critically important signals takes place:
- Costimulation: the CD40 molecule on the B cell membrane binds to the CD154 ligand expressed on the activated T helper cell. This provides the B cell with a powerful stimulus for survival and preparation for division.
- Cytokine support: the T cell secretes cytokines (primarily IL-4, as well as IL-2 and IL-5), receptors for which appear on the B cell. It is cytokines that propel the lymphocyte through the cell cycle and trigger mitosis.
Further fate: follicular and extrafollicular pathways
After receiving all signals, activated B cells can follow one of two developmental pathways. The choice depends on the expression of the CXCR5 chemokine receptor:
- Extrafollicular pathway (CXCR5⁻): lymphocytes remain outside the follicles, migrate to medullary cords, and rapidly turn into short-lived plasma cells. They predominantly produce IgM. Isotype switching or an increase in antibody affinity for the antigen does not occur here.
- Follicular pathway (CXCR5⁺): cells rush inside the primary follicles following a CXCL13 chemokine gradient. Here, germinal centers are formed. In close interaction with follicular dendritic cells and specialized T helper cells (Tfh), B cells divide intensively, undergo heavy chain isotype switching and affinity maturation, becoming long-lived producers of highly specific antibodies.
Briefly: answers to topical questions
Which morphological structures are involved in antigen presentation by a B lymphocyte to a T helper cell?
Antigen presentation by a B lymphocyte to a T helper cell involves a specialized zone of intercellular contact — the immunological synapse, which includes the supramolecular activation cluster (SMAC).
Main interaction molecules:
- MHC-II–peptide on the B cell — the complex recognized by the TCR of the T helper cell;
- TCR on the T helper cell — recognizes the «MHC-II–peptide» complex;
- CD40 on the B cell and CD154 on the activated T cell;
- B7 (CD80/CD86) on the B cell and CD28 on the T cell.
Class and exam questions
Why does a B lymphocyte need contact with a T helper cell?
Antigen binding provides only the first signal. For full activation, expansion, and differentiation, a B cell requires a costimulatory signal (CD40–CD154 interaction) and cytokines (IL-4, IL-5), which only a T helper cell provides.
How are the complement system and B cell activation connected?
The receptor-antigen complex activates complement, resulting in the formation of the C3d fragment. It binds to the CD21 molecule on the B cell surface, repeatedly amplifying the intracellular activation signal.
What is the essence of the follicular pathway of B lymphocyte development?
Cells expressing CXCR5 migrate into lymphoid follicles, where they form germinal centers. There, they undergo antibody isotype switching and affinity maturation, turning into long-lived antibody producers.
What else to study on this topic
- Detailed structure of the secondary follicle (light and dark zones of the germinal center).
- Mechanisms of immunoglobulin heavy chain isotype switching.
- The role of follicular dendritic cells (FDCs) in follicle morphogenesis.
- Intracellular BCR cascades: PLCγ, Ras-MAPK, and PKC pathways.
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