Learning Objective
Differentiate the primary roles of B and T lymphocytes in adaptive immunity, and correlate their mechanisms with clinical conditions such as hypersensitivity reactions and organ rejection.
Overview of Adaptive Immunity
Adaptive immunity is mediated primarily by B cells and T cells, which originate from bone marrow stem cells but mature in different organs:
- B cells mature in the Bone marrow
- T cells mature in the Thymus
| Cell Type | Immunity Type | Maturation Site | Key Function |
|---|---|---|---|
| B cell | Humoral | Bone marrow | Antibody production |
| T cell | Cell-mediated | Thymus | Cytotoxicity, immune regulation |
B Cells: Humoral Immunity
Functions
| Function | Description | Clinical Relevance |
|---|---|---|
| Antigen Recognition & Presentation | Recognize and present antigen via MHC II to helper T cells. | Defective presentation → poor antibody response |
| Somatic Hypermutation | Improves antigen specificity of immunoglobulins. | Basis of affinity maturation |
| Antibody Production | Differentiate into plasma cells → secrete antibodies (IgM → IgG, IgA, IgE). | Important in vaccine response |
| Immunologic Memory | Memory B cells persist → rapid response upon re-exposure. | Basis for booster vaccination |
Key Point:
B cells provide long-term protection through antibody production and memory formation.
T Cells: Cell-Mediated Immunity
Subtypes and Functions
| Subtype | Function | Mechanism | Clinical Correlate |
|---|---|---|---|
| CD4⁺ Helper T cells | Help B cells make antibodies; secrete cytokines to activate macrophages and other immune cells. | Recognize antigen presented on MHC II. | Deficiency → HIV/AIDS immunodeficiency |
| CD8⁺ Cytotoxic T cells | Kill virus-infected and tumor cells via perforin and granzymes. | Recognize antigen presented on MHC I. | Tumor immune surveillance |
| Regulatory T cells (Tregs) | Suppress immune response, maintain tolerance. | Express CD25 and FoxP3. | Dysfunction → autoimmunity |
T Cell–Mediated Responses
- Type IV Hypersensitivity (e.g., contact dermatitis, TB skin test)
- Acute and Chronic Organ Rejection (cellular rejection mediated by T cells)
Key Point:
T cells are critical for directly attacking infected or abnormal cells and coordinating immune responses.
Key Differences Summary
| Feature | B Cells | T Cells |
|---|---|---|
| Immunity Type | Humoral | Cell-mediated |
| Effector Mechanism | Antibody secretion | Cytotoxicity, cytokine release |
| Antigen Presentation | MHC II to CD4⁺ T cells | MHC I to CD8⁺ T cells |
| Memory | Memory B cells | Memory T cells |
| Hypersensitivity | Type I, II, III | Type IV |
| Involved in | Vaccination, antibody-mediated diseases | Viral infections, graft rejection |
Key Takeaways
- B cells → antibody-mediated defense (extracellular pathogens).
- T cells → cellular defense (intracellular pathogens).
- CD4⁺ T cells assist immune orchestration; CD8⁺ T cells perform cytotoxic killing.
- Type IV hypersensitivity and graft rejection are T-cell–mediated responses.
