The immune system protects the body from infections and foreign substances. The major cells involved in immunity are white blood cells or leukocytes, all of which originate from hematopoietic stem cells in the bone marrow. Among these, T cells and B cells are the key players of adaptive immunity.
Other Immune Cells (Brief Overview)
Granulocytes
- Neutrophils: Most abundant WBCs; excellent at phagocytosis; rapidly reach inflammation sites.
- Eosinophils: Important in defense against worms and play major roles in allergies and asthma.
- Basophils: Participate in allergic responses and parasite defense.
Monocytes–Macrophages
- Monocytes circulate in blood and become macrophages in tissues.
- Macrophages clear dead cells, debris and detect early infection.
- They release cytokines, recruit leukocytes and present antigens to T cells.
Dendritic Cells
- Found in tissues; excellent antigen-presenting cells.
- Engulf pathogens, then migrate to lymph nodes to activate T cells.
Mast Cells
- Located in connective tissues; contain histamine and TNF-α.
- Play central roles in allergic (type I hypersensitivity) reactions.
Natural Killer (NK) Cells
- Kill virus-infected and tumor cells.
- Part of innate immunity but similar in appearance to lymphocytes.
Lymphocytes – Key Cells of Adaptive Immunity
Lymphocytes include T cells and B cells. Each cell carries unique antigen receptors, meaning every T or B cell recognizes only one specific antigen.
B Lymphocytes (B Cells)
B cells are responsible for humoral immunity (antibody-mediated defense).
B-Cell Receptor (BCR)
- Consists of a membrane-bound antibody plus accessory proteins (Igα and Igβ).
- The antibody is Y-shaped with antigen-binding sites at the tips.
Activation and Response
Each B cell has only one antigen specificity. When it binds to its specific antigen:
- It undergoes clonal expansion (produces more identical B cells).
- It differentiates into plasma cells, which secrete large amounts of free antibodies.
Types of B Cells
- Naive B cells: Mature B cells that have not yet met their antigen.
- Memory B cells: Long-lived cells formed after an immune response; quickly respond if the antigen reappears.
T Lymphocytes (T Cells)
T cells are responsible for cell-mediated immunity.
T-Cell Receptor (TCR)
- Made of two chains (usually α and β).
- Always associated with CD3 complex, which transmits activation signals.
- Each T cell has a single antigen specificity.
MHC and Antigen Presentation
T cells recognize antigens only when they are presented with MHC molecules on another cell.
Class I MHC
- Presents antigens from inside the cell (e.g., viruses).
- Found on all nucleated cells.
- Presents to CD8+ cytotoxic T cells (TC cells).
- Genes: HLA-A, HLA-B, HLA-C.
Class II MHC
- Presents antigens from outside the cell (phagocytosed material).
- Present on professional antigen-presenting cells: B cells, macrophages, dendritic cells.
- Presents to CD4+ helper T cells (TH cells).
- Genes: HLA-DP, HLA-DQ, HLA-DR.
Subsets of T Cells
1. Helper T Cells (TH cells – CD4+)
Activated by antigen presented on Class II MHC. Two major subtypes:
- TH1 cells: Release IL-2, IFN-γ; activate macrophages, promote inflammation and help in cell-mediated immunity.
- TH2 cells: Release IL-4, IL-5; promote antibody production and anti-inflammatory responses.
2. Cytotoxic T Cells (TC cells – CD8+) Activated by Class I MHC. Their job is to: Kill virus-infected cells Kill tumor cells Induce apoptosis in target cells
3. Regulatory T Cells (TREG – CD4+)
Responsible for immune suppression and prevention of autoimmunity. They release IL-4, IL-10 and TGF-β to suppress overactive immune responses.
4. Naive T Cells
Mature T cells that have not yet encountered their specific antigen. They become activated after proper antigen presentation and costimulation.
T Cell Activation and Responses
When a T cell binds its specific antigen presented in the correct MHC context:
- It undergoes clonal expansion.
- It differentiates into effector T cells (TH1, TH2, CTLs).
- Cytokines released by helper T cells regulate inflammation, antibody production and cell killing.
Detailed Notes:
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