Innate and Adaptive Immunity: Cells, Barriers and Effector Mechanisms

Innate and adaptive immunity are complementary arms of host defence. Innate immunity acts within minutes through barriers, pattern-recognition receptors, soluble proteins and rapid effector cells. Adaptive immunity develops more slowly during a first encounter, but it recognises specific antigens, generates antibodies and T-cell responses, and remembers previous exposure. Most effective immune responses require both systems working together.

Learning outcomes

The learner should be able to define innate and adaptive immunity, describe physical and chemical barriers, explain pattern recognition and complement, identify major immune cells, compare humoral and cellular responses, explain antigen presentation and immune memory, and relate dysregulation to infection, sepsis, allergy, autoimmunity and immunodeficiency.

1. The layered organisation of immunity

1. Barrier defence: skin, mucosa, mucus, cilia, secretions, microbiota and mechanical clearance prevent entry.
2. Innate recognition and inflammation: resident cells detect danger, release mediators and recruit effectors.
3. Innate killing: complement, phagocytosis, antimicrobial peptides, oxidative mechanisms and NK-cell cytotoxicity limit spread.
4. Adaptive activation: dendritic cells present antigen and activate lymphocytes in lymphoid organs.
5. Resolution and memory: inflammation contracts, tissue repairs and memory cells remain.

2. Physical and chemical barriers

Barrier	Protective mechanism	Failure consequence
Skin	Keratin, tight junctions, dryness, acidic lipids and antimicrobial peptides	Cellulitis, wound infection and invasive disease after barrier disruption
Respiratory mucosa	Mucus, ciliary clearance, cough, macrophages and airway peptides	Pneumonia and infection when cilia or cough are impaired
Gastrointestinal tract	Acid, bile, peristalsis, mucus, microbiota and secretory IgA	Enteric infection or dysbiosis
Urinary/reproductive tract	Urine flow, mucosal epithelium, pH and microbiota	Urinary and genital infection
Eye	Tears, blinking, lysozyme and intact cornea	Keratitis or conjunctival infection
Blood-brain barrier	Endothelial tight junctions and specialised transport	Neuroinvasion or difficult antimicrobial penetration

3. Innate immune recognition

3.1 PAMPs and DAMPs

Pathogen-associated molecular patterns (PAMPs) are conserved microbial structures such as lipopolysaccharide, flagellin, unmethylated nucleic acid or double-stranded viral RNA. Damage-associated molecular patterns (DAMPs) are host molecules released by necrotic or stressed cells. Both can activate inflammation; therefore, sterile trauma and infection may look similar early.

3.2 Pattern-recognition receptors

• Toll-like receptors: detect extracellular or endosomal microbial components.
• NOD-like receptors: detect cytosolic bacterial products and cellular stress; some form inflammasomes.
• RIG-I-like receptors: recognise viral RNA in the cytosol.
• C-type lectin receptors: detect fungal and other carbohydrate patterns.
• cGAS–STING pathway: detects cytosolic DNA and induces interferon responses.

Receptor activation induces NF-kappa B, interferon-regulatory factors, inflammasomes and cytokine production. Excessive activation causes tissue injury, systemic inflammatory response and shock.

4. Innate immune cells

Cell	Main functions	Important clinical clues
Neutrophil	Rapid chemotaxis, phagocytosis, granules, reactive oxygen species and extracellular traps	Neutropenia causes severe bacterial/fungal infection; excessive activation injures tissue
Monocyte/macrophage	Phagocytosis, cytokines, antigen presentation, tissue repair and resolution	Persistent macrophage activation contributes to chronic inflammation
Dendritic cell	Captures antigen and primes naive T cells; links innate to adaptive immunity	Essential for vaccine and antiviral responses
Natural killer cell	Kills stressed, infected or tumour cells; produces interferon-gamma	Recognises reduced MHC I and antibody-coated targets
Mast cell	Histamine, lipid mediators and cytokines; barrier defence and allergy	Anaphylaxis, urticaria and parasite responses
Eosinophil	Helminth killing and allergic inflammation	Parasitic disease, asthma and drug reactions
Basophil	Circulating type-2 effector cell and histamine source	Allergic and helminth-related responses

5. Innate effector mechanisms

5.1 Phagocytosis

1. Chemotaxis directs cells toward complement fragments, chemokines and microbial products.
2. Adhesion and recognition occur through pattern receptors and opsonins such as IgG and C3b.
3. The pathogen is engulfed into a phagosome.
4. Fusion with lysosomes, acidification, proteases, reactive oxygen and nitrogen species destroy the target.
5. Debris is cleared and antigen may be presented to lymphocytes.

5.2 Complement

Pathway	Trigger	Important products
Classical	Antibody bound to antigen or selected acute-phase proteins	C3b opsonisation, C3a/C5a inflammation and membrane attack complex
Lectin	Microbial carbohydrates recognised by lectin	Same central cascade without pre-existing antibody
Alternative	Spontaneous C3 activation amplified on unprotected microbial surfaces	Rapid opsonisation and lysis

C3b coats microbes, C3a and C5a promote inflammation, C5a recruits neutrophils, and the membrane attack complex can lyse susceptible organisms. Complement is controlled by host regulators; uncontrolled activation can damage self-tissue.

5.3 Interferons and antiviral defence

Type I interferons induce an antiviral state, increase antigen presentation and activate NK cells. Cytokines coordinate fever, acute-phase responses, leukocyte recruitment and adaptive differentiation. Excessive cytokine release can cause capillary leak, hypotension, organ dysfunction and shock.

6. Adaptive immunity

6.1 B lymphocytes and humoral immunity

B cells recognise native antigen through the B-cell receptor. With T-cell help, they proliferate, class-switch, affinity-mature and differentiate into antibody-secreting plasma cells or memory B cells. Antibodies neutralise toxins and viruses, opsonise microbes, activate complement and support antibody-dependent cellular cytotoxicity.

6.2 T lymphocytes and cellular immunity

• CD4 helper T cells: coordinate immune cells through cytokines and contact signals.
• CD8 cytotoxic T cells: kill infected, malignant or otherwise abnormal cells presenting antigen on MHC I.
• Regulatory T cells: limit activation and maintain tolerance.
• Memory T cells: respond quickly during re-exposure.

6.3 Antigen presentation

MHC I on nearly all nucleated cells presents intracellular peptides to CD8 cells. MHC II on professional antigen-presenting cells presents extracellularly derived peptides to CD4 cells. Cross-presentation allows dendritic cells to prime CD8 responses against some extracellular or tumour antigens.

7. Innate–adaptive crosstalk

• Dendritic cells and cytokines determine the type of T-cell response.
• Complement fragments enhance B-cell activation and antibody responses.
• Antibodies coat pathogens so neutrophils, macrophages and NK cells can clear them.
• Helper T cells activate macrophages and support B-cell class switching.
• Inflammation creates the vascular and cellular environment needed for adaptive cells to enter tissue.
• Regulatory mechanisms prevent the response from continuing after the threat has been removed.

8. Innate versus adaptive comparison

Domain	Innate	Adaptive
Specificity	Shared molecular patterns	Unique antigen epitopes
Receptors	Germline encoded PRRs	Somatically rearranged BCR/TCR
Speed	Immediate	Delayed at first exposure
Memory	Limited/trained features in some cells	Strong, durable antigen-specific memory
Effectors	Barriers, complement, phagocytes, NK cells	Antibodies, helper and cytotoxic T cells
Typical failure	Sepsis, uncontrolled inflammation or barrier infection	Immunodeficiency, autoimmunity or poor vaccine protection

9. Tolerance and protection from self-attack

Central tolerance removes or edits many self-reactive lymphocytes in thymus and bone marrow. Peripheral tolerance uses anergy, regulatory cells, inhibitory receptors and deletion. Innate checkpoints also restrict inflammation. Failure of these mechanisms contributes to autoimmune disease; excessive tolerance or immune suppression permits infection and cancer.

10. Clinical consequences of immune imbalance

10.1 Immunodeficiency

• Barrier defects cause recurrent skin or mucosal infection.
• Neutrophil defects predispose to bacterial/fungal infection and poor wound healing.
• Complement deficiency increases susceptibility to selected bacteria and immune-complex disease.
• B-cell deficiency causes recurrent encapsulated-bacterial infection.
• T-cell deficiency causes viral, fungal, protozoal and opportunistic infection.
• Combined deficiency is life-threatening in infancy or after immunosuppression.

10.2 Hypersensitivity and allergy

Exaggerated responses may be immediate IgE/mast-cell reactions, antibody-mediated injury, immune-complex disease or T-cell-mediated delayed hypersensitivity. Anaphylaxis is an emergency regardless of the original trigger.

10.3 Autoimmunity

Adaptive lymphocytes and antibodies attack self-tissues when tolerance fails. Innate cytokines and complement amplify injury. Examples include systemic lupus, rheumatoid arthritis, autoimmune thyroid disease and type 1 diabetes.

10.4 Sepsis and systemic inflammation

Pathogen and tissue signals activate PRRs, complement, coagulation and endothelium. Dysregulated inflammation produces vasodilation, capillary leak, microthrombi and organ dysfunction. A high or low temperature and a high or low white count can occur; sepsis is a clinical emergency, not a laboratory label alone.

11. Emergency medicine applications

12. Factors influencing immune responses

• Age, genetics, nutrition, pregnancy and microbiome.
• Pathogen dose, route, replication strategy and immune evasion.
• Vaccination, previous infection and antigenic variation.
• Sleep, stress, tobacco, alcohol, chronic disease and metabolic state.
• Corticosteroids, chemotherapy, immunosuppressants, biologics and HIV.
• Barrier integrity, vascular supply and the local tissue environment.

13. Cases

Case 1: Neutropenic fever

A patient receiving chemotherapy presents with fever but little local inflammation. Neutrophil deficiency can blunt innate signs. Treat as a time-critical infection, obtain appropriate cultures and follow the local neutropenic sepsis protocol.

Case 2: Recurrent meningococcal disease

Repeated invasive infection with a compatible organism raises concern for terminal complement deficiency or another immune problem. Treat the acute episode first, then arrange immunology assessment and preventive planning.

Case 3: Severe sepsis after trauma

Damaged tissue releases DAMPs that activate innate receptors even alongside infection. Control bleeding and source, resuscitate and treat infection; sterile inflammation does not exclude sepsis.

14. Exam pearls

• Innate immunity is rapid and pattern-based; adaptive immunity is specific and memory-based.
• Dendritic cells are the key bridge from tissue danger to naive T-cell activation.
• Complement opsonises, recruits inflammation and can lyse susceptible organisms.
• CD4 cells coordinate; CD8 cells kill infected or abnormal cells; B cells produce antibodies.
• Excessive immune activation causes tissue damage, while insufficient activation causes infection.
• Always stabilise the emergency before pursuing detailed immunological classification.

References and further reading

• NIAID: Features of an immune response.
• NCBI Bookshelf: Innate immunity.
• NCBI Bookshelf: Innate immune receptors.
• NCBI Bookshelf: Complement and innate immunity.
• NCBI Bookshelf: Adaptive recognition and effector mechanisms.
• Follow local sepsis, anaphylaxis, immunisation and immunodeficiency protocols.