Paper I
Question
Discuss mechanisms of innate immunity.
Answer
Innate immunity is the body’s first line of defence — non-specific, present from birth, acts rapidly (minutes to hours), and lacks immunological memory.
Mechanisms:
- Physical/mechanical barriers — intact skin, mucous membranes, mucociliary clearance, flushing action of tears/saliva/urine.
- Chemical/physiological barriers — gastric acid, lysozyme (digests bacterial peptidoglycan), antimicrobial peptides (defensins), normal commensal flora (competitive exclusion).
- Cellular components:
- Phagocytes (neutrophils, macrophages) — engulf and destroy microorganisms via phagocytosis, respiratory burst, and lysosomal enzymes.
- Natural killer (NK) cells — kill virus-infected and tumour cells without prior sensitization, via “missing self” recognition and perforin/granzyme-mediated killing.
- Dendritic cells — phagocytose antigen and bridge to adaptive immunity via antigen presentation.
- Mast cells, eosinophils, basophils — involved in inflammatory and antiparasitic responses.
- Soluble/humoral components:
- Complement system — activated via classical, alternative, or lectin pathways, leading to opsonization (C3b), inflammation (anaphylatoxins C3a/C5a), and direct lysis (membrane attack complex).
- Acute-phase proteins (e.g., CRP) — rise during inflammation, enhance opsonization.
- Interferons (type I, IFN-α/β) — induce an antiviral state in neighbouring cells.
- Pattern recognition — innate immune cells recognize conserved microbial structural patterns (PAMPs — LPS, peptidoglycan, viral dsRNA) via germline-encoded pattern recognition receptors (PRRs), notably Toll-like receptors (TLRs), triggering inflammatory cytokine release and initiating the adaptive immune response.
- Inflammatory response — vasodilation, increased vascular permeability, and leukocyte recruitment to the site of infection/injury, mediated by cytokines and chemokines.
Significance: innate immunity provides immediate initial defence and shapes/directs the subsequent adaptive (antigen-specific) response via cytokine signalling and antigen presentation, working in an integrated fashion with adaptive immunity.