Paper I
Question
Innate immunity
Answer
Innate immunity is the body’s first line of defence against infection — present from birth, non-specific (does not target a particular pathogen), acts rapidly (within minutes to hours), and lacks immunological memory (its response does not improve with repeated exposure to the same pathogen), in contrast to the slower, antigen-specific, memory-generating adaptive immune system.
Components:
- Physical/mechanical barriers — intact skin, mucous membranes, mucociliary clearance in the respiratory tract, and the flushing action of tears/saliva/urine.
- Chemical/physiological barriers — gastric acid, lysozyme in secretions (saliva, tears — digests bacterial peptidoglycan), antimicrobial peptides (defensins), and normal commensal flora (competitive exclusion of pathogens).
- Cellular components — phagocytes (neutrophils, macrophages), which engulf and destroy microorganisms; natural killer (NK) cells, which kill virus-infected and tumour cells without prior sensitization; dendritic cells, which, besides phagocytosis, serve as the crucial bridge to adaptive immunity by presenting processed antigen to T cells.
- Soluble/humoral components — the complement system (activated via classical, alternative, or lectin pathways, leading to opsonization, inflammation, and direct lysis via the membrane attack complex), acute-phase proteins (e.g., CRP), and interferons (which induce an antiviral state in neighbouring cells).
- Pattern recognition — innate immune cells recognize broad, conserved microbial structural patterns (Pathogen-Associated Molecular Patterns, PAMPs — e.g., bacterial LPS, peptidoglycan, viral double-stranded RNA) via germline-encoded pattern recognition receptors (PRRs), notably Toll-like receptors (TLRs), triggering an inflammatory response.
Relationship to adaptive immunity: innate immunity provides the immediate initial defence and also shapes and directs the subsequent adaptive (antigen-specific) immune response, via cytokine signalling and antigen presentation by dendritic cells — the two systems work in an integrated, complementary fashion rather than in isolation.