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Antigen presenting cells
Related Subjects: |Major Histocompatibility complex |Immune response |Complement |Opsonisation |Toll-like Receptors (TLRs) |Immunoglobulins |Immunology |Antigen presenting cells |T lymphocytes
🧬 Physiology
- Antigen-presenting cells (APCs) activate T cells by displaying processed antigens on their surface.
- 🔑 T cells only recognize antigens when presented via the Major Histocompatibility Complex (MHC).
- MHC proteins are coded by Human Leucocyte Antigen (HLA) genes on chromosome 6.
- Class I MHC → intracellular antigens | Class II MHC → extracellular antigens.
- CD markers classify T cells (e.g. CD4, CD8), each with a distinct immune function.
🧑🤝🧑 CD4 T Cells (Helper T Cells)
- Recognize antigens on MHC Class II (from extracellular pathogens).
- 📢 Release cytokines (e.g. IL-2, IFN-γ) to activate B cells, macrophages, and CD8 T cells.
- 🦠 In HIV infection, CD4 count falls → impaired immunity and ↑ infection risk.
🗡️ CD8 T Cells (Cytotoxic T Cells)
- Recognize antigens on MHC Class I (from intracellular pathogens like viruses).
- Destroy infected, tumour, or graft cells using perforins and apoptosis pathways.
🏷️ Class I MHC Molecules
- Found on nearly all nucleated cells (❌ not RBCs).
- Associated with β₂-microglobulin, presenting intracellular peptides to CD8 T cells.
- ⚔️ Abnormal or infected cells are destroyed when CD8 T cells detect these antigens.
📦 Class II MHC Molecules
- Expressed only on professional APCs (B cells, macrophages, dendritic cells, monocytes).
- Present extracellular antigens to CD4 T cells, triggering wider immune activation.
- CD4 T cells split into two functional subsets:
- Th1 → IL-2, IFN-γ, TNF-α → ⏱ delayed-type hypersensitivity & cell-mediated immunity.
- Th2 → IL-4, IL-5, IL-10 → 🤧 allergic responses & antibody production.
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