Innate & Adaptive Immune Responses

Innate & Adaptive Immunity

Innate Immunity

• Speed: immediate (minutes–hours); Specificity: broad pattern recognition
• Pattern recognition receptors (PRRs): Toll-like receptors (TLRs) recognise PAMPs (pathogen-associated molecular patterns)
• Key cells: neutrophils (first responders), macrophages, NK cells, dendritic cells
• Complement: classical (antibody-mediated), lectin (MBL), alternative (spontaneous) → C3b opsonisation, MAC lysis, C5a chemotaxis

Adaptive Immunity

• Speed: days–weeks (primary), faster on re-exposure (memory)
• Specificity: antigen-specific via TCR (T cells) and BCR/antibody (B cells)

T Cell Subsets

| Subset | CD marker | Function |

|--------|-----------|----------|

| Cytotoxic T cell | CD8 | Kill virus-infected/tumour cells via perforin/granzyme |

| T helper 1 | CD4 | Activate macrophages (IFN-╬│), help CD8, cell-mediated |

| T helper 2 | CD4 | Help B cells (IL-4, IL-5, IL-13), allergy, helminths |

| Treg | CD4/CD25/FOXP3 | Suppress immune responses, prevent autoimmunity |

B Cells & Antibodies

• B cell activation: antigen + T helper 2 cell (CD40L–CD40, IL-4) → plasma cell → antibody
• Immunoglobulins: IgM (primary response, agglutination), IgG (secondary, crosses placenta), IgA (mucosal), IgE (allergy, parasites)
• Somatic hypermutation (affinity maturation) and class switching in germinal centres

MHC & Antigen Presentation

• MHC I (HLA-A/B/C): on all nucleated cells; presents endogenous (intracellular) antigens to CD8 T cells
• MHC II (HLA-DR/DP/DQ): on APCs (dendritic cells, macrophages, B cells); presents exogenous antigens to CD4 T cells