14.6 Innate Immunity: Barriers, Complement & Cellular Effectors

Medical College Admission Test MCAT

Track your progress as you read

Finished sections get a check mark.

1 MCAT Architecture, Test-Day Logistics & Scientific Inquiry Skills

1.1 MCAT Architecture, Test-Day Logistics & Scoring Scale (472–528) 1.2 Scientific Inquiry & Reasoning Skills (SIRS 1–4) & Experimental Design 1.3 Passage Navigation, Section Pacing & Strategic Guessing

2 Physics: Mechanics, Fluids & Periodic Motion

2.1 Translational Kinematics, Projectile Motion & Newton's Laws 2.2 Friction, Centripetal Force & Rotational Equilibrium in Biomechanics 2.3 Mechanical Work, Pressure-Volume Work & Forms of Energy 2.4 The Work-Energy Theorem, Power & Mechanical Advantage 2.5 Hydrostatics, Fluid Dynamics & Circulatory Hemodynamics 2.6 Periodic Motion, Wave Fundamentals, Superposition & Standing Waves 2.7 Sound Intensity, the Decibel Scale & the Doppler Effect

3 Physics: Electromagnetism, Optics & Atomic Physics

3.1 Coulomb's Law, Electric Fields & Electric Potential 3.2 Electric Dipoles, Membrane Potentials & the Nernst Equation 3.3 Electric Current, Resistance & Resistor Networks 3.4 Capacitance, Dielectrics & Axonal Cable Theory 3.5 Light, Refraction, Mirrors & Thin Lenses 3.6 The Optical System of the Eye & Corrective Lens Prescriptions 3.7 Nuclear Architecture, Binding Energy & Radioactive Decay Modes 3.8 Decay Kinetics, Half-Life & Medical Radiation Applications

4 General Chemistry: Atoms, Solutions & Thermodynamics

4.1 Quantum Numbers, Electron Configurations & Periodic Trends 4.2 Lewis Structures, Formal Charge, VSEPR Geometry & Molecular Polarity 4.3 States of Matter, Phase Diagrams, Heating Curves & Solvation 4.4 Concentration Units, Henry's Law & Colligative Properties 4.5 Thermodynamic Systems, the First Law, Enthalpy & Entropy 4.6 Gibbs Free Energy, Equilibrium & Bioenergetic Reaction Coupling 4.7 Collision Theory, Transition States & Empirical Rate Laws 4.8 Integrated Rate Laws, Half-Life, Arrhenius Behaviour & Catalysis

5 General Chemistry: Equilibrium, Acids, Bases & Electrochemistry

5.1 Chemical Equilibrium, Equilibrium Constants & Le Chatelier's Principle 5.2 Acid-Base Definitions, pH Scale, Titrations & Physiological Buffers 5.3 Electrochemical Cells, Reduction Potentials & the Nernst Equation

6 Organic Chemistry: Structure, Stereochemistry & Mechanisms

6.1 IUPAC Nomenclature, Functional Group Priority & Orbital Hybridisation 6.2 Resonance, Inductive Effects & Conformational Analysis 6.3 The Isomer Hierarchy, Chirality & CIP Priority Rules 6.4 Optical Activity, Diastereomers, Meso Compounds & Fischer Projections 6.5 Carbonyl Reactivity: Aldehydes, Ketones & Nucleophilic Additions 6.6 Carboxylic Acids & Derivatives: Nucleophilic Acyl Substitution

7 Bio-Organic Reactions, Laboratory Separations & Spectroscopy

7.1 Alcohol Physical Properties, Acidity, Leaving Groups & Protecting Groups 7.2 Carbon Oxidation States, Laboratory Oxidations & Biological Redox Cofactors 7.3 Extraction, Acid-Base Separation & Distillation 7.4 Analytical and Biomacromolecular Chromatography 7.5 Infrared and UV-Vis Spectroscopy 7.6 1H-NMR, 13C-NMR & Integrated Spectral Elucidation

8 Biochemistry: Amino Acids, Peptides & Enzymes

8.1 Amino Acid Stereochemistry & Side-Chain Classification 8.2 Zwitterions, Titration Curves, Isoelectric Points & the Peptide Bond 8.3 The Hierarchy of Protein Structure 8.4 Folding Thermodynamics, Haemoglobin Cooperativity & Denaturation 8.5 Catalytic Mechanisms, Enzyme Classification & Michaelis-Menten Kinetics 8.6 Enzyme Inhibition Modes & Allosteric Cooperativity

9 Biochemistry: Cellular Metabolism & Bioenergetics

9.1 Glycolysis: The Ten Steps, Regulation & Anaerobic Fermentation 9.2 Gluconeogenesis Bypass Enzymes & the Pentose Phosphate Pathway 9.3 The Pyruvate Dehydrogenase Complex & the Citric Acid (Krebs) Cycle 9.4 Mitochondrial Architecture & the Respiratory Complexes 9.5 Chemiosmotic Coupling, ATP Synthase, NADH Shuttles & Uncouplers 9.6 Fatty Acid Beta-Oxidation, Ketogenesis & Glycogen Dynamics

10 Molecular Biology: Nucleic Acids & Genetic Technologies

10.1 Nucleic Acid Chemistry, the Double Helix & Chromatin Organisation 10.2 DNA Replication: Replisome Enzymology & Fork Dynamics 10.3 DNA Damage & Cellular Repair Pathways 10.4 The Central Dogma, Strand Nomenclature & Transcription Machinery 10.5 Post-Transcriptional Processing of Eukaryotic Pre-mRNA 10.6 Regulation of Gene Expression: Operons, Enhancers & Epigenetics 10.7 The Genetic Code, Wobble & Mutation Taxonomy 10.8 tRNA Charging & the Stages of Ribosomal Translation 10.9 Translation Inhibitors & Post-Translational Processing 10.10 Restriction Enzymes, Cloning Vectors, cDNA Libraries & PCR 10.11 Blotting, Sequencing, Microarrays & CRISPR-Cas9

11 Cellular Biology & Microbiology

11.1 Eukaryotic Organelles, Membrane Transport & Cytoskeletal Dynamics 11.2 The Eukaryotic Cell Cycle, Mitosis, Checkpoints & Apoptosis 11.3 Cell Theory, Bacterial Morphology, Genetics & Sub-Viral Agents

12 Genetics, Evolution & Embryology

12.1 Mendel's Laws & the Cytology of Meiosis 12.2 Genetic Crosses, Non-Mendelian Inheritance, Linkage & Pedigree Analysis 12.3 The Hardy-Weinberg Principle, Assumptions & Clinical Calculations 12.4 Mechanisms of Evolution, Speciation & Macroevolutionary Patterns 12.5 Gametogenesis, Fertilisation, Cleavage & Implantation 12.6 Gastrulation, Germ Layer Derivatives, Stem Cell Potency & Neurulation

13 Human Physiology: Control & Transport Systems

13.1 Neuronal Anatomy, the Resting Potential & the Action Potential 13.2 Conduction Velocity, Synaptic Transmission & Postsynaptic Summation 13.3 Neurotransmitter Systems, CNS/PNS Organisation & Reflex Arcs 13.4 Hormone Classification & the Hypothalamus-Pituitary Axis 13.5 Negative Feedback Architecture, Endocrine Glands & Calcium Homeostasis 13.6 Cardiac Anatomy, Electrophysiology, the Cardiac Cycle & Cardiac Output 13.7 Vascular Hemodynamics, Starling Forces, Blood Components & Hemostasis 13.8 Respiratory Anatomy, Ventilation Mechanics & Pulmonary Surfactant 13.9 Spirometry, Dead Space, Gas Exchange & Haemoglobin Allostery 13.10 Carbon Dioxide Transport, the Chloride Shift & Acid-Base Control of Breathing

14 Human Physiology: Maintenance, Defense & Locomotion

14.1 Renal Anatomy, Glomerular Ultrafiltration & Tubular Transport 14.2 The Countercurrent Multiplier & Endocrine Regulation of Renal Function 14.3 GI Architecture, the Enteric Nervous System & Gastric Secretion 14.4 Pancreatic and Biliary Digestion & Intestinal Nutrient Absorption 14.5 Hepatic Processing, GI Endocrine Regulators & the Colon 14.6 Innate Immunity: Barriers, Complement & Cellular Effectors 14.7 Adaptive Humoral Immunity: B Cells & Immunoglobulins 14.8 Adaptive Cellular Immunity: T Cells, Thymic Selection & MHC Presentation 14.9 Muscle Types, Sarcomere Anatomy & Excitation-Contraction Coupling 14.10 Bone Architecture, Remodelling & the Integumentary System

15 CARS: Passage Analysis & Argument Mapping

15.1 CARS Architecture, Passage Genres (Humanities vs Social Sciences) & Mindset 15.2 Active Reading, Author's Central Thesis & Tone Analysis 15.3 Rhetorical Structure, Claim-Evidence Linkages & Argument Mapping

16 CARS: Question Types & Reasoning Strategies

16.1 Skill 1 Demands, Question Stems, Detail Retrieval & Paraphrase Recognition 16.2 Inferential Meaning, Distractor Taxonomy & a Worked Skill 1 Passage 16.3 Skill 2 Architecture, Question Stems, Unstated Assumptions & Evidence Strength 16.4 Rhetorical Mechanisms, Logical Fallacies & a Worked Skill 2 Passage 16.5 Skill 3 Architecture, Question Stems, Application & Analogy Mapping 16.6 Incorporating New Information, Calibrating Evidentiary Impact & a Worked Skill 3 Passage

17 Sensory Processing, Perception & Cognition

17.1 Sensation versus Perception & Sensory Receptor Classification 17.2 Psychophysical Thresholds, Weber's Law, Adaptation & Signal Detection Theory 17.3 Vision: Ocular Anatomy, Phototransduction & Central Visual Pathways 17.4 Audition & the Vestibular System 17.5 Somatosensation, Pain & Chemoreception 17.6 Models of Attention & Working Memory Architecture 17.7 Long-Term Memory Taxonomy, Encoding, Storage & Retrieval 17.8 Forgetting, Interference, Memory Distortion & Long-Term Potentiation 17.9 Problem Solving, Heuristics, Cognitive Biases & Language 17.10 Sleep Architecture, Circadian Neurobiology, Sleep Disorders & Psychoactive Drugs

18 Emotion, Stress, Motivation & Personality

18.1 Components of Emotion & Classical Theories of Emotion 18.2 The Limbic Circuit, Stress Appraisal & Selye's General Adaptation Syndrome 18.3 Intrinsic and Extrinsic Motivation, Drive-Reduction, Arousal & Incentive Theories 18.4 Cognitive and Humanistic Need Models & the Neuroendocrine Control of Hunger and Sex 18.5 The Psychoanalytic Perspective & Neo-Freudian Revisions 18.6 Humanistic, Trait & Social-Cognitive Perspectives on Personality 18.7 Biopsychosocial Etiology, Schizophrenia, Mood & Anxiety Disorders 18.8 Somatic, Dissociative, Personality & Neurodegenerative Disorders

19 Learning, Social Behavior & Identity

19.1 Non-Associative Learning & Classical Conditioning 19.2 Operant Conditioning, Reinforcement Schedules & Observational Learning 19.3 Self-Concept & Lifespan Developmental Theories 19.4 Kohlberg's Moral Development & Sociocultural Frameworks 19.5 Social Influence: Conformity, Obedience, Group Dynamics & Prosocial Behavior 19.6 Social Cognition, Attribution Theory, Prejudices, Stereotypes & Discrimination

20 Sociology, Demographics & Healthcare Disparities

20.1 Macro versus Micro Paradigms, Structural Functionalism & Conflict Theory 20.2 Symbolic Interactionism, Social Constructionism & Modern Paradigms 20.3 Major Social Institutions: Structure, Function & Dysfunction 20.4 Demographics, Population Pyramids, the Demographic Transition & Globalisation 20.5 Systems of Stratification, Socioeconomic Status & Social Mobility 20.6 Poverty, Spatial Inequality, the Socioeconomic Gradient & Health Inequities

14.6 Innate Immunity: Barriers, Complement & Cellular Effectors

Key Takeaways Explain This Topic AI

The innate immune system delivers immediate, antigen-non-specific defense via physical/chemical barriers, the complement cascade (opsonin C3b, anaphylatoxins C3a/C5a, and membrane attack complex C5b-C9), and pattern recognition receptors (PRRs/TLRs) recognizing conserved microbial PAMPs.

Natural killer (NK) cells provide innate surveillance against viral infection and neoplastic transformation by executing apoptosis (via perforin and granzymes) upon detecting downregulation of surface MHC Class I molecules ('missing self' recognition) or through antibody-dependent cellular cytotoxicity (ADCC).

Innate and adaptive immunity differ on three axes the MCAT tests directly: innate responses act within minutes to hours using germline-encoded pattern recognition receptors and generate no memory, whereas adaptive responses take days to develop, use somatically rearranged receptors with exquisite antigen specificity, and produce memory cells that make the secondary response faster and stronger.

Last updated: September 2026

14.6 Innate Immunity: Barriers, Complement & Cellular Effectors

Note

Foundational Standard : The mammalian immune system discriminates self from non-self, neutralizes pathogenic microorganisms, and eliminates malignant cells. The MCAT requires a comprehensive understanding of the structural and temporal dichotomy between innate and adaptive immunity, the biochemical cascade of the complement system, natural killer cell surveillance, B-cell activation, somatic hypermutation and antibody isotypes, T-cell thymic ontogeny, and the intracellular mechanics of MHC Class I versus Class II antigen processing and presentation.

The Architectural Dichotomy: Innate vs. Adaptive Immunity

Host defense is organized into two cooperating arms differing in speed, specificity, and immunological memory:

Innate Immunity : Immediate to early response (operational within 0–12 hours of exposure). It is antigen-non-specific, does not require prior sensitization, and does not alter its kinetics or magnitude upon repeated exposures ( lacks immunological memory ). It utilizes germline-encoded Pattern Recognition Receptors (PRRs) , such as Toll-Like Receptors (TLRs) , to detect conserved microbial structural motifs known as Pathogen-Associated Molecular Patterns (PAMPs) (e.g., bacterial lipopolysaccharide [LPS], flagellin, peptidoglycan, viral double-stranded RNA) and host-derived Damage-Associated Molecular Patterns (DAMPs) (e.g., HMGB1, extracellular ATP, uric acid crystals).

Adaptive Immunity : Delayed initial response (takes 3–7 days for clonal expansion and differentiation). It is exquisitely antigen-specific, generates a diverse repertoire of antigen receptors via somatic gene rearrangement, and develops long-lived memory cells that mediate a rapid, high-affinity secondary response upon re-exposure.

Innate Defenses: Anatomical Barriers, Chemical Factors & Complement Innate Physical & Chemical Defenses:

- Keratinized Stratified Squamous Epidermis: Physical impermeable barrier; continuous desquamation.

- Mucociliary Escalator: Respiratory goblet cells trap particles in mucus; ciliated cells sweep them upward.

- Lysozyme (Tears, Saliva, Mucus): Hydrolyzes beta-1,4-glycosidic bonds between NAM and NAG in bacterial peptidoglycan.

- Acidic Secretions: Gastric hydrochloric acid (pH 1.5-2.0) and vaginal lactic acid (pH ~4.0) kill swallowed/colonizing microbes.

- Antimicrobial Peptides: Defensins and cathelicidins insert into microbial membranes, forming lethal pores.

The Complement Cascade

The complement system comprises over 30 heat-labile plasma proteins synthesized primarily by the liver. They circulate as inactive zymogens that activate through an amplifying proteolytic cascade along three distinct initiation pathways:

Classical Pathway : Initiated when the C1 complex ( C 1 q , C 1 r , C 1 s C1q, C1r, C1s C 1 q , C 1 r , C 1 s ) binds directly to the Fc region of antibody-antigen immune complexes containing either IgM (most efficient; a single pentameric IgM can bind C1q) or aggregated IgG (requires at least two adjacent IgG monomers).

Lectin Pathway : Initiated when mannose-binding lectin (MBL) or ficolins bind conserved terminal mannose residues on foreign bacterial or fungal cell surfaces, activating MBL-associated serine proteases (MASPs).

Alternative Pathway : Initiated spontaneously at low basal rates by the continuous "tickover" (hydrolysis) of plasma C3 into C 3 ( H 2 O ) C3(H_2O) C 3 ( H 2 ​ O ) , which binds Factor B and Factor D on microbial surfaces lacking host regulatory proteins (such as CD55/DAF and CD59). Complement Cascade Convergence & Effector Outputs:

All three pathways assemble a C3 Convertase:

• Classical/Lectin: C4b2a
• Alternative: C3bBb

C3 Convertase cleaves abundant plasma C3 into:

-> C3a: Small peptide acting as a potent Anaphylatoxin (induces mast cell degranulation, histamine release, vasodilation).

-> C3b: Binds covalently to pathogen surfaces, acting as a master Opsonin recognized by CR1 receptors on macrophages.

C3b joins the convertase to assemble C5 Convertase (C4b2a3b or C3bBbC3b), cleaving C5 into:

-> C5a: Potent Anaphylatoxin and the strongest chemoattractant for neutrophil recruitment and extravasation.

-> C5b: Initiates Membrane Attack Complex (MAC) assembly: sequentially recruits C6, C7, C8, and 10-16 C9 monomers.

Polymerized C9 forms a 10 nm transmembrane aqueous pore: Membrane Attack Complex (MAC, C5b-C9).

Fluid rushes in down the osmotic gradient, causing cell swelling and osmotic lysis of Gram-negative bacteria and enveloped viruses.

Type I Interferons (IFN-α and IFN-β)

When viral pattern molecules (e.g., intracellular dsRNA) engage cytosolic RIG-I-like receptors or endosomal TLR3/TLR7, virally infected host cells synthesize and secrete Type I Interferons (IFN- α \alpha α and IFN- β \beta β ) . These cytokines act in an autocrine and paracrine manner, binding IFNAR receptors on adjacent uninfected cells to activate the JAK-STAT pathway, transcriptionally upregulating hundreds of interferon-stimulated genes (ISGs) :

Protein Kinase R (PKR) : Phosphorylates elongation factor eIF-2 α \alpha α , halting all host and viral mRNA translation.

2'-5'-Oligoadenylate Synthetase (OAS) and RNase L : Degrades viral and host single-stranded RNA.

MHC Class I Upregulation : Increases surface MHC I display on uninfected and infected cells, enhancing recognition by C D 8 + CD8^+ C D 8 + cytotoxic T cells.

Innate Cellular Effectors

Macrophages : Derived from circulating blood monocytes that extravasate into tissues, differentiating into specialized resident phagocytes (e.g., Kupffer cells in the liver, microglia in the CNS, alveolar macrophages in the lungs, osteoclasts in bone). Phagocytose opsonized microbes into phagosomes , which fuse with lysosomes to form phagolysosomes . Inside, they execute an oxygen-dependent respiratory burst : O 2 → NADPH Oxidase ⋅ O 2 − (Superoxide) → Superoxide Dismutase H 2 O 2 → Myeloperoxidase (MPO) HOCl (Hypochlorous Acid / Bleach) \text{O}_2 \xrightarrow{\text{NADPH Oxidase}} \cdot\text{O}_2^- \text{ (Superoxide)} \xrightarrow{\text{Superoxide Dismutase}} \text{H}_2\text{O}_2 \xrightarrow{\text{Myeloperoxidase (MPO)}} \text{HOCl (Hypochlorous Acid / Bleach)} O 2 ​ NADPH Oxidase ​ ⋅ O 2 − ​ (Superoxide) Superoxide Dismutase ​ H 2 ​ O 2 ​ Myeloperoxidase (MPO) ​ HOCl (Hypochlorous Acid / Bleach) Macrophages also secrete key pro-inflammatory cytokines: IL-1 (fever, osteoclast activation), TNF- α \alpha α (vascular permeability, cachexia), IL-6 (acute-phase reactant synthesis by hepatocytes), IL-8 (neutrophil chemotaxis), and IL-12 ( T h 1 T_h1 T h ​ 1 differentiation).

Neutrophils (Polymorphonuclear Leukocytes / PMNs) : Most abundant circulating white blood cells (~60%–70%). Rapid first responders to acute inflammation. They extravasate across inflamed post-capillary venules via a multi-step adhesion cascade: Margination → \to → Rolling (endothelial E-selectin and P-selectin bind neutrophil Sialyl Lewis X) → \to → Tight Adhesion (endothelial ICAM-1 binds activated neutrophil β 2 \beta_2 β 2 ​ -integrins [LFA-1/Mac-1]) → \to → Diapedesis (transmigration via PECAM-1/CD31) → \to → Chemotaxis (migrating toward C5a, LTB4, IL-8, or bacterial fMLP). Neutrophils phagocytose, degranulate antimicrobial enzymes, and cast Neutrophil Extracellular Traps (NETs) —webs of extracellular chromatin laced with granule proteins that trap and kill extracellular bacteria.

Eosinophils : Bilobed granulocytes specialized in defending against large multicellular parasites ( helminths ) that cannot be phagocytosed. Activated by IL-5 from T h 2 T_h2 T h ​ 2 cells; bind IgE-coated parasites via Fc receptors and release Major Basic Protein (MBP) and eosinophil cationic protein to induce tegument damage. Prominent in allergic and asthmatic reactions.

Basophils and Mast Cells : Tissue-resident mast cells (skin, mucosa) and circulating basophils express high-affinity F c ϵ R I Fc\epsilon RI F cϵ R I receptors pre-loaded with monomeric IgE . When multivalent allergen cross-links adjacent surface-bound IgE molecules, it triggers explosive degranulation, releasing pre-formed histamine (vasodilation, bronchoconstriction, increased vascular permeability), heparin, and newly synthesized lipid mediators ( Leukotrienes L T C 4 , L T D 4 , L T E 4 LTC_4, LTD_4, LTE_4 L T C 4 ​ , L T D 4 ​ , L T E 4 ​ ).

Natural Killer (NK) Cells : Innate lymphocytes (CD56+, CD16+, CD3-) that survey host cells for viral infection and malignant transformation without prior clonal expansion. Regulated by the "Missing Self" Hypothesis :

Healthy host cells express high levels of surface MHC Class I . This binds NK cell Killer-cell Inhibitory Receptors (KIRs) , delivering a dominant inhibitory signal that overrides activating signals.

Virally infected cells or neoplastic cells frequently downregulate surface MHC Class I to hide from C D 8 + CD8^+ C D 8 + cytotoxic T cells. In the absence of MHC Class I engagement, the inhibitory signal is abolished, and activating receptors trigger NK degranulation.

Cytotoxic Mechanism : NK cells release perforin (forms transmembrane pores in the target membrane) and granzymes (serine proteases that enter through perforin pores to cleave and activate initiator caspase-3 and BID, triggering caspase-dependent apoptosis).

Antibody-Dependent Cellular Cytotoxicity (ADCC) : NK cells express CD16 (an F c γ R I I I Fc\gamma RIII F c γ R III receptor). When IgG antibodies bind foreign antigens on an infected target cell, CD16 binds the exposed IgG Fc domains, triggering targeted perforin/granzyme degranulation.

Dendritic Cells (DCs) : The quintessential professional Antigen-Presenting Cells (APCs) bridging innate and adaptive immunity. Reside as immature sentinels in peripheral tissues (e.g., cutaneous Langerhans cells ). Upon engulfing pathogens via phagocytosis or macropinocytosis, they mature, upregulate costimulatory molecules ( CD80/CD86 ) and CCR7 chemokine receptors, and migrate via afferent lymphatics to regional lymph nodes to present processed peptides on MHC molecules to naive T cells.

Innate Cellular Effectors

Each innate leukocyte is defined by what it kills and how, and several double as the bridge into adaptive immunity.

Cell Principal function Distinguishing feature

Neutrophil Rapid phagocytosis of bacteria and fungi Most abundant circulating leukocyte; first responder to infection

Macrophage Phagocytosis, cytokine release, antigen presentation Presents on MHC class II, linking innate to adaptive immunity

Dendritic cell Professional antigen presentation Migrates to lymph nodes to prime naive T cells

Natural killer cell Apoptosis of virus-infected and tumour cells Kills targets that have downregulated MHC class I; uses perforin and granzymes

• Eosinophil Defence against multicellular parasites Also drives late-phase allergic inflammation
• Basophil and mast cell Histamine release Bind IgE; mediate immediate hypersensitivity