What are the differences in mechanism between T cells (T lymphocytes) and B cells (B lymphocytes) in the immune response?

T Cell vs B Cell Mechanisms in Immune Response

Fundamental Distinction

T cells mediate cell-mediated immunity through direct cellular interactions and cytokine production, while B cells mediate humoral immunity through antibody production and secretion. 1, 2

T Cell Mechanisms

Development and Maturation

• T cell precursors originate in bone marrow as hematopoietic stem cells, then migrate to the thymus where they undergo T cell-specific differentiation and acquire the T cell receptor (TCR). 1, 2
• The thymic microenvironment directs precursor cells toward T cell differentiation, where the TCR becomes modified and tailored for specific antigens. 1
• After thymic maturation, T cells circulate through blood and secondary lymphoid tissues (lymph nodes, spleen) searching for antigens presented by antigen-presenting cells (APCs). 1

Functional Mechanisms

• T cells recognize antigens only when presented by APCs in the context of MHC molecules—this is called MHC restriction. 1

• Upon activation by antigen recognition, T cells proliferate and differentiate into specialized subsets with distinct functions:

  • Th1 cells magnify inflammation through soluble protein secretion (particularly IFN-γ) and macrophage stimulation 1
  • Th2 cells stimulate B lymphocytes to mature and produce antibodies 1
  • Th17 cells produce IL-17, IL-17F, and IL-22 for inflammatory responses 1
  • Cytotoxic CD8+ T cells directly eliminate infected or malignant cells 1
  • Regulatory T cells (Tregs) modulate immune responses 1

• T cells control viral infections through two pathways: direct cytopathic elimination of infected cells and non-cytopathic cytokine-mediated suppression of viral replication. 1

B Cell Mechanisms

Development and Maturation

• B cells develop in bone marrow and express surface immunoglobulin (BCR) that serves as their antigen receptor, with each B cell expressing a unique BCR specific for particular antigens. 2
• Normal peripheral blood B cells typically represent more than 3% of lymphocytes in healthy individuals. 2
• B cells differentiate into distinct subsets including marginal zone B cells, switched memory B cells, transitional B cells, and plasma cells, each with specialized functions. 2

Functional Mechanisms

• B cells function as professional antigen-presenting cells—they use their surface immunoglobulin to bind, internalize, process, and present antigen to T helper cells in an MHC-restricted manner. 3, 4
• The surface immunoglobulin allows specific B cells to concentrate antigen with very high efficiency, making them 100-10,000 times more efficient at presenting their cognate antigen compared to non-specific APCs. 4
• B cells require T cell help for antibody production through two mechanisms: lymphokines (growth and differentiation factors) and contact-dependent signals that enable B cells to respond to lymphokines. 3
• The CD40 molecule on B cells interacts with CD40 ligand on activated T helper cells, providing critical contact-dependent help. 3

Antibody Production

• Upon receiving T cell help, B cells proliferate and differentiate into plasma cells that secrete antibodies. 5
• Antibodies mediate humoral immunity by neutralizing pathogens, opsonizing targets for phagocytosis, and activating complement. 5
• B cells can also produce cytokines, but unlike T cells that produce multiple cytokines upon activation, B cells require specific differentiation and activation conditions to produce cytokines. 6

Critical Interdependence

B Cells Help T Cells

• B cells contribute to T cell diversity and function—mice lacking B cells have profoundly contracted TCR repertoires (0.08% of normal) and significantly delayed allograft rejection. 7
• B cells can transfer antigen to macrophages through BCR-mediated uptake, focusing immunity toward specific antigens and enabling other APCs to activate CD4+ T cells. 8

T Cells Help B Cells

• T helper cells are absolutely required for B cell responses to protein antigens—this interaction requires antigen-specific recognition where B cells present antigen to T cells via MHC class II molecules. 3, 4
• Th2 cells specifically stimulate B cell maturation and antibody production. 1

Immunosuppression Effects

Impact on Both Cell Types

• Immunosuppressive regimens (corticosteroids, calcineurin inhibitors, antimetabolites) impair both T and B cell responses by blocking cellular proliferation after antigen stimulation and inhibiting cytokine production. 1
• Corticosteroids inhibit IL-1, IL-2, IL-6, TNF, and IFN-γ production, blocking antigen-induced T cell proliferation. 1
• Calcineurin inhibitors directly inhibit IL-2-dependent T cell proliferation and, by blocking IL-4 and IL-5 production, have secondary inhibitory effects on B cell function and antibody production. 1

Clinical Implications

• Rituximab specifically depletes B cells by targeting CD20, resulting in depletion of circulating and tissue-based B cells within 2 weeks, with recovery beginning at approximately 6 months. 5
• B cell depletion reduces immunoglobulin levels (particularly IgM) and certain inflammatory markers (IL-6, CRP, RF, anti-CCP). 5

Key Clinical Distinctions

• T cells provide immune surveillance and direct cellular immunity, making them critical for controlling intracellular pathogens and tumor cells. 1
• B cells provide long-term humoral immunity through antibody production, making them critical for neutralizing extracellular pathogens and toxins. 5
• Memory T and B cells persist long-term, with memory cell recall being more effective than primary vaccination in immunosuppressed patients. 1