Inflammation, Fever, and Complement - Anatomy and Physiology

SECTION 1

Historical Context & Motivation

The study of inflammation stretches back to antiquity, when Roman physician Aulus Cornelius Celsus first catalogued the four cardinal signs of inflammation—rubor (redness), tumor (swelling), calor (heat), and dolor (pain)—in the first century CE. For nearly two millennia, clinicians recognized these signs without understanding the cellular and molecular machinery responsible for them. The emergence of microscopy, germ theory, and immunochemistry gradually revealed that inflammation, fever, and the complement system are interlocking arms of innate immunity, the body's rapid, non-specific first line of defense against infection and tissue damage.

~30 CE • Cardinal Signs of Inflammation

Celsus describes rubor, tumor, calor, dolor in De Medicina, establishing a clinical framework for inflammation that persists today.

1882 • Metchnikoff & Phagocytosis

Élie Metchnikoff observes starfish larvae engulfing foreign particles, coining the term phagocytosis and demonstrating that mobile cells actively defend the host.

1895 • Discovery of Complement

Jules Bordet identifies a heat-labile serum factor that 'complements' antibody-mediated bacterial lysis. This factor is later named the complement system.

1927 • Pyrogenic Cytokines

Research reveals that fever is not merely a symptom but an active physiological response mediated by endogenous pyrogens (later identified as IL-1, IL-6, and TNF-α) acting on the hypothalamus.

1980s–Present • Molecular Immunology Era

Toll-like receptors, inflammasomes, and the three complement activation pathways are characterized, revealing the precise molecular links between pathogen detection, inflammation, and fever.

A central question has driven immunology for over a century: how does the body mount an immediate, coordinated defense against pathogens it has never encountered before? The answer lies in the interplay of inflammation, fever, and complement—three mechanisms that detect danger signals, recruit immune cells, and create hostile conditions for microbial invaders, all within minutes to hours of initial exposure. This lesson examines each mechanism in detail, then integrates them into a unified model of innate immune defense.

SECTION 2

Core Principles & Definitions

Inflammation, fever, and complement share a common purpose—defending the body against threats—but each accomplishes this through distinct cellular and molecular strategies. Before exploring their mechanisms in depth, it is essential to establish the foundational concepts that unite these three innate immune responses. Together, they illustrate a principle that recurs throughout physiology: rapid, non-specific defense mechanisms buy time for the slower, antigen-specific adaptive immune system to mount a tailored response.

Inflammation

A localized vascular and cellular response to tissue injury or infection. Vasodilation and increased capillary permeability allow plasma proteins and leukocytes to enter damaged tissue, producing the classic signs of redness, swelling, heat, pain, and loss of function.

Fever (Pyrexia)

A systemic elevation of the body's thermoregulatory set point, mediated by pyrogens acting on the hypothalamic thermostat. Fever enhances immune cell activity, inhibits pathogen replication, and increases the production of acute-phase proteins.

Complement System

A cascade of over 30 plasma proteins that, when activated, promote opsonization, chemotaxis, inflammation amplification, and direct pathogen lysis via the membrane attack complex (MAC).

Pattern Recognition

Innate immune cells express pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs) that detect conserved microbial structures called pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).

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KEY TAKEAWAY

Think of innate immunity as a building's integrated security system. Inflammation is the alarm that seals off corridors and summons guards (leukocytes) to the breach. Fever is like raising the building's temperature to make conditions intolerable for intruders while energizing the security team. The complement system functions as an automated defense grid—motion-sensor turrets that tag intruders (opsonization), sound additional alarms (chemotaxis), and even punch holes in them directly (MAC lysis). All three work in parallel and reinforce one another, ensuring no single point of failure in the defense.

SECTION 3

The Inflammatory Response — Visual Explanation

The inflammatory response unfolds in a predictable sequence of vascular and cellular events. When tissue is injured or invaded by pathogens, resident sentinel cells—primarily macrophages and mast cells—detect PAMPs and DAMPs through their TLRs and release chemical mediators such as histamine, prostaglandins, and cytokines (including IL-1, IL-6, and TNF-α). These mediators trigger arteriolar vasodilation, increase capillary permeability, and establish a chemotactic gradient that guides circulating neutrophils and monocytes toward the site of damage. The diagram below illustrates this cascade from initial injury through leukocyte extravasation.

The acute inflammatory response in four stages. (1) Tissue injury or pathogen invasion activates resident mast cells (MC, gold) and macrophages (Mϕ, green). (2) These sentinel cells release histamine, prostaglandins, and cytokines (IL-1, TNF-α), causing local vasodilation and increased capillary permeability. (3) Neutrophils (N, violet) undergo margination, rolling, adhesion, and diapedesis to exit the bloodstream. (4) Chemotactic gradients guide neutrophils to the site where they phagocytose pathogens (red).

Observe in the diagram that the process is sequential yet rapid. Within minutes of injury, histamine from mast cells causes local arteriolar vasodilation and venular permeability changes, producing the hallmark redness and swelling. Cytokines released by tissue macrophages upregulate selectins and integrins on the endothelial surface, which act as molecular 'Velcro' to capture fast-flowing neutrophils from the bloodstream—a process called margination. Once adhered, neutrophils squeeze between endothelial cells via diapedesis and migrate along the chemotactic gradient of complement fragments (C3a, C5a) and bacterial products to reach the infection site. This orchestrated recruitment ensures that the immune system's most abundant phagocytes arrive precisely where they are needed.

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Fifth Cardinal Sign

Rudolf Virchow added a fifth cardinal sign—functio laesa (loss of function)—in the 19th century. Swelling and pain reflexively limit movement of an injured area, which protects it from further damage and facilitates healing. All five signs are direct consequences of the vascular and cellular events depicted above.

SECTION 4

Mechanism of Fever

While inflammation is primarily a localized response, fever represents a systemic escalation of innate defense. Fever is not simply an elevated body temperature; it is a tightly regulated upward reset of the hypothalamic thermoregulatory set point, orchestrated by signaling molecules called pyrogens. Pyrogens fall into two categories: exogenous pyrogens (microbial products such as lipopolysaccharide from Gram-negative bacteria) and endogenous pyrogens (host cytokines—primarily IL-1, IL-6, and TNF-α—released by activated macrophages and other leukocytes). Exogenous pyrogens trigger the release of endogenous pyrogens, which then travel through the bloodstream to the preoptic area of the hypothalamus.

The Prostaglandin E₂ Pathway

When endogenous pyrogens reach the hypothalamus, they stimulate the enzyme cyclooxygenase-2 (COX-2) in perivascular cells of the blood-brain barrier, catalyzing the synthesis of prostaglandin E₂ (PGE₂). PGE₂ binds to EP3 receptors on thermosensitive neurons, raising the thermoregulatory set point. The body then perceives its current temperature as 'too cold' and activates heat-generating mechanisms—vasoconstriction to reduce heat loss, shivering thermogenesis to generate metabolic heat, and behavioral responses such as seeking warmth and curling up. This is why patients with rising fevers often feel chills: the set point has shifted upward, but core temperature has not yet caught up.

FEVER SIGNALING CASCADE

Exogenous Pyrogen → Macrophage activation → IL-1 / IL-6 / TNF-α → COX-2 → PGE₂ → Hypothalamic set point ↑

COX-2 = cyclooxygenase-2; PGE₂ = prostaglandin E₂. Antipyretics such as aspirin and ibuprofen work by inhibiting COX enzymes, thereby blocking PGE₂ synthesis and lowering the set point back toward 37 °C.

Benefits and Risks of Fever

Enhanced immune function: Moderate fever (38–39 °C) increases neutrophil migration, macrophage phagocytic activity, and lymphocyte proliferation.
Inhibited pathogen replication: Many bacteria and viruses replicate optimally at 37 °C; even a 1–2 °C rise can reduce their growth rate significantly.
Acute-phase protein synthesis: The liver increases production of C-reactive protein (CRP), fibrinogen, and complement components during fever.
Danger zone: Temperatures above 41 °C (hyperpyrexia) risk protein denaturation, seizures, and multi-organ failure—fever must be self-limiting.

Fever resolution, or defervescence, occurs when pyrogen levels decline as the infection is controlled. The set point returns to normal, and the body now perceives itself as 'too hot,' triggering vasodilation and sweating to dissipate excess heat. This is why the 'breaking' of a fever is accompanied by profuse perspiration.

SECTION 5

The Complement System — Pathways & Outcomes

The complement system consists of roughly 30 plasma proteins, most synthesized by the liver, that circulate in inactive (zymogen) forms. Upon activation, they undergo sequential proteolytic cleavage in a cascade that amplifies the signal enormously—a single initiating event can generate millions of effector molecules within minutes. Three distinct pathways converge on a common step: the cleavage of C3 into C3a and C3b. From this convergence point, the cascade proceeds to form the membrane attack complex and release potent inflammatory mediators.

The three complement activation pathways—classical (triggered by antigen-antibody complexes), lectin (triggered by mannose-binding lectin), and alternative (triggered by spontaneous C3 hydrolysis on pathogen surfaces)—all converge on C3 convertase, which cleaves C3 into C3a and C3b. C3b opsonizes pathogens and contributes to C5 convertase formation. C3a and C5a serve as anaphylatoxins that amplify inflammation and chemotaxis. The terminal pathway assembles the membrane attack complex (MAC), which lyses target cells.

Pathway	Trigger	Key Components	Innate or Adaptive?
Classical	Antigen-antibody (IgG/IgM) complexes bind C1q	C1q, C1r, C1s, C4, C2	Bridges adaptive → innate
Lectin	Mannose-binding lectin (MBL) or ficolins bind carbohydrates on pathogen surfaces	MBL, MASP-1/2, C4, C2	Innate
Alternative	Spontaneous hydrolysis of C3 (tick-over); amplified on surfaces lacking regulatory proteins	C3, Factor B, Factor D, Properdin	Innate

Comparison of the three complement activation pathways

A critical feature of complement regulation is that host cells express complement regulatory proteins such as decay-accelerating factor (DAF/CD55), membrane cofactor protein (MCP/CD46), and protectin (CD59) on their surfaces. These molecules accelerate the degradation of C3 convertase or block MAC assembly, ensuring that the destructive power of complement is directed at pathogens and foreign cells rather than healthy host tissue. Deficiencies in these regulators underlie diseases such as paroxysmal nocturnal hemoglobinuria (PNH), in which host red blood cells are lysed by uncontrolled complement activation.

SECTION 6

Worked Example — Tracing an Innate Immune Response

Consider the following clinical scenario: a patient sustains a deep splinter wound on the sole of the foot while walking barefoot outdoors. The wound is contaminated with soil bacteria, including Gram-negative species that possess lipopolysaccharide (LPS) in their outer membranes. Let us trace the innate immune response step by step, integrating inflammation, complement activation, and fever.

Tracing the Innate Response to a Contaminated Splinter Wound

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Step 1 — Pathogen Detection

Resident macrophages and dendritic cells in the dermis detect bacterial LPS via TLR-4 (a pattern recognition receptor specific for Gram-negative endotoxin). Damaged epithelial cells release DAMPs, including ATP and HMGB1, which further activate sentinel cells.

TLR-4 engagement initiates NF-κB signaling → cytokine gene transcription
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Step 2 — Inflammatory Mediator Release

Activated macrophages secrete IL-1, IL-6, TNF-α, and chemokines (e.g., IL-8/CXCL8). Mast cells in the connective tissue degranulate, releasing histamine and heparin. Together, these mediators cause arteriolar vasodilation, increased venular permeability, and upregulation of E-selectin and ICAM-1 on endothelial cells.

Local redness, heat, swelling, and pain develop within minutes
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Step 3 — Complement Activation

Plasma proteins leak into the tissue through permeable capillaries. The alternative complement pathway activates spontaneously on bacterial surfaces lacking DAF and CD59. Simultaneously, mannose-binding lectin recognizes mannose residues on bacterial cell walls, activating the lectin pathway. Both pathways generate C3 convertase, producing C3b (opsonin) and C3a/C5a (anaphylatoxins that further amplify inflammation and recruit more phagocytes).

Bacteria are coated with C3b; MAC assembly begins on Gram-negative outer membranes
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Step 4 — Neutrophil Recruitment & Phagocytosis

Neutrophils circulating in post-capillary venules detect chemotactic signals (C5a, IL-8) and begin margination (rolling along the endothelium via selectin-carbohydrate interactions), followed by firm adhesion (integrin-ICAM-1 binding) and diapedesis. Upon reaching the wound, neutrophils recognize C3b-opsonized bacteria via complement receptor 1 (CR1) and phagocytose them, killing bacteria within phagolysosomes using reactive oxygen species and defensins.

Massive neutrophil influx produces pus (dead neutrophils, bacteria, and tissue debris)
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Step 5 — Systemic Fever Response

IL-1 and IL-6 enter the bloodstream and reach the preoptic area of the hypothalamus, where they stimulate COX-2 to produce PGE₂. PGE₂ resets the thermoregulatory set point upward. The patient experiences chills and shivers, core temperature rises to approximately 38.5 °C, and the liver increases synthesis of acute-phase proteins (CRP, complement components, fibrinogen).

Systemic fever enhances immune function while inhibiting bacterial growth
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Step 6 — Resolution

As bacteria are cleared, PAMP/DAMP levels decline and macrophages switch from producing pro-inflammatory to anti-inflammatory mediators (IL-10, TGF-β, lipoxins, resolvins). Neutrophil recruitment ceases, and apoptotic neutrophils are cleared by macrophages (efferocytosis). Fibroblasts begin tissue repair. The hypothalamic set point returns to 37 °C, vasodilation and sweating dissipate excess heat, and the fever breaks.

Tissue repair proceeds; homeostasis is restored

SECTION 7

Comparing Innate Defense Mechanisms

Although inflammation, fever, and complement all serve defensive functions, they differ substantially in scope, speed, specificity, and potential for causing host damage. Understanding these distinctions is essential for clinical reasoning—many pathologies arise not from immune failure but from excessive or dysregulated activation of these very systems.

Feature	Inflammation	Fever	Complement
Scope	Local (acute); can become systemic (SIRS/sepsis)	Systemic (whole-body thermoregulatory reset)	Local (at pathogen surface); systemic effects via anaphylatoxins
Speed of onset	Seconds to minutes	Minutes to hours	Seconds (alternative) to hours (classical)
Key mediators	Histamine, prostaglandins, IL-1, TNF-α, leukotrienes	IL-1, IL-6, TNF-α → PGE₂	C3a, C3b, C5a, C5b-9 (MAC)
Primary function	Contain damage, recruit leukocytes, initiate repair	Inhibit microbial growth, enhance immune cell function	Opsonize, lyse, amplify inflammation, recruit phagocytes
Pathological excess	Chronic inflammation (e.g., rheumatoid arthritis)	Hyperpyrexia (>41 °C), febrile seizures	PNH, complement-mediated hemolytic uremic syndrome (HUS)
Pharmacological targets	NSAIDs (COX inhibitors), corticosteroids, anti-TNF biologics	Antipyretics (aspirin, acetaminophen, ibuprofen)	Eculizumab (anti-C5 monoclonal antibody)

Comparative overview of inflammation, fever, and complement

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CLINICAL SIGNIFICANCE

The same mechanisms that protect us can become pathological when dysregulated. Sepsis illustrates this principle powerfully: an overwhelming infection triggers systemic inflammation (SIRS), disseminated complement activation, and high fevers. The resulting vasodilation, capillary leak, and coagulopathy can lead to multi-organ failure—essentially, the innate immune system's collateral damage exceeds the pathogen's destructive potential. Modern critical care focuses on supporting organ function while modulating—not eliminating—the immune response, recognizing that these defenses are essential when properly calibrated.

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Connections to Adaptive Immunity & Advanced Topics

Innate immunity does not operate in isolation—it fundamentally shapes and activates the adaptive immune response. The bridge between innate and adaptive immunity is largely constructed by dendritic cells, which are activated during inflammation, process antigens at the site of infection, and migrate to regional lymph nodes where they present peptide-MHC complexes to naïve T cells. The cytokine milieu established by innate inflammation (e.g., IL-12, IL-6, type I interferons) determines which subset of T helper cells differentiates—Th1, Th2, or Th17—thereby shaping the character of the subsequent adaptive response. Complement fragment C3d, covalently bound to antigen, also lowers the activation threshold for B cells by cross-linking CR2 (CD21) with the B cell receptor, acting as a molecular adjuvant.

Feature	Innate Immunity	Adaptive Immunity
Response time	Immediate (minutes to hours)	Delayed (days to weeks for primary response)
Specificity	Broad (PAMPs/DAMPs); limited receptor diversity (~10³ PRRs)	Highly specific (unique epitopes); vast receptor diversity (~10¹¹ TCRs/BCRs)
Memory	No classical memory (though 'trained immunity' is now recognized)	Immunological memory via memory T and B cells
Key effectors	Neutrophils, macrophages, NK cells, complement, cytokines	T cells (CD4⁺, CD8⁺), B cells, antibodies
Complement link	Alternative and lectin pathways are fully innate	Classical pathway is activated by antibodies (IgG, IgM)

Innate vs. adaptive immunity

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Looking Ahead: Trained Immunity

Recent research has revealed that innate immune cells such as monocytes and NK cells can undergo epigenetic reprogramming after an initial infection, leading to enhanced responses to subsequent, unrelated infections. This phenomenon, termed trained immunity, blurs the traditional boundary between innate and adaptive immunity. It is mediated by histone modifications and metabolic rewiring rather than gene rearrangement, and may explain some of the non-specific protective effects of certain vaccines (e.g., BCG).

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain why a patient with a rising fever experiences chills and shivering, even though their core body temperature is already above 37 °C. What specific physiological mechanism accounts for this apparent paradox?

Show answer

During fever onset, endogenous pyrogens (IL-1, IL-6, TNF-α) stimulate the synthesis of PGE₂ in the hypothalamus, which resets the thermoregulatory set point upward—for example, to 39 °C. Because the patient's actual core temperature has not yet reached this new set point, the hypothalamus interprets the current temperature (e.g., 37.5 °C) as 'below normal.' The body then activates heat-generating and heat-conserving mechanisms: cutaneous vasoconstriction reduces radiative heat loss, and skeletal muscle shivering generates metabolic heat. The patient subjectively perceives coldness—hence the chills—despite an objectively elevated temperature. This continues until core temperature matches the new set point.

Question 2: BASIC IDENTIFICATION

Match each complement fragment to its primary function: (a) C3b, (b) C5a, (c) C5b-9 (MAC). Functions: opsonization, anaphylatoxin/chemotaxis, direct pathogen lysis.

Show answer

(a) C3b → Opsonization. C3b covalently binds to pathogen surfaces and is recognized by complement receptor 1 (CR1) on phagocytes, marking the pathogen for engulfment. (b) C5a → Anaphylatoxin/chemotaxis. C5a is the most potent anaphylatoxin; it triggers mast cell degranulation, increases vascular permeability, and serves as a powerful chemotactic signal for neutrophils. (c) C5b-9 (MAC) → Direct pathogen lysis. The membrane attack complex inserts into the target cell membrane to form a transmembrane pore, allowing uncontrolled influx of water and ions that leads to osmotic lysis.

Question 3: INTERMEDIATE

A researcher finds that a patient's serum can lyse antibody-coated sheep red blood cells (the CH50 assay) but fails to lyse zymosan-coated particles (the AH50 assay, which tests the alternative pathway). C3 levels are normal. Which component(s) of the complement system are most likely deficient? Explain your reasoning.

Show answer

Since the classical pathway (CH50) is functional and C3 levels are normal, the deficiency must lie in a component unique to the alternative pathway. The alternative pathway requires Factor B, Factor D, and Properdin, none of which are used by the classical or lectin pathways. A deficiency in Factor B or Factor D would prevent formation of the alternative pathway C3 convertase (C3bBb), thereby abolishing alternative pathway-mediated lysis while leaving the classical pathway intact. Properdin deficiency is also possible—Properdin stabilizes the C3bBb convertase, and its absence markedly reduces alternative pathway efficiency. Genetic testing for these factors would confirm the diagnosis.

Question 4: APPLIED

A patient with paroxysmal nocturnal hemoglobinuria (PNH) presents with dark-colored urine in the morning and chronic anemia. PNH results from a deficiency in GPI-anchored proteins, including CD55 (DAF) and CD59 (protectin), on red blood cells. Explain the pathophysiology: why are these patients' RBCs lysed by complement, and why is hemolysis worse at night?

Show answer

In PNH, a somatic mutation in the PIGA gene prevents synthesis of glycosylphosphatidylinositol (GPI) anchors, so complement regulatory proteins CD55 and CD59 are absent from affected RBCs. CD55 normally accelerates decay of C3 convertase on host cell surfaces, and CD59 blocks insertion of C9 into the MAC. Without these regulators, low-level spontaneous complement activation (tick-over) on RBC surfaces proceeds unchecked, leading to MAC assembly and intravascular hemolysis. Hemolysis is classically worse at night because slight respiratory acidosis during sleep lowers blood pH, which enhances complement activation. The released hemoglobin is filtered by the kidneys, producing the characteristic hemoglobinuria (dark urine) that is most concentrated in the first morning void. Treatment with eculizumab (anti-C5) blocks MAC formation and dramatically reduces hemolysis.

Question 5: CRITICAL THINKING

Some clinical guidelines recommend against routinely treating moderate fevers (38–39 °C) in otherwise healthy patients with uncomplicated infections. Others argue that antipyretics improve patient comfort and may be warranted. Construct a physiological argument for each position, drawing on the mechanisms of fever discussed in this lesson. Under what circumstances might each approach be more appropriate?

Show answer

The argument for tolerating moderate fever rests on its documented immunological benefits: fever enhances neutrophil chemotaxis and phagocytosis, increases lymphocyte proliferation, elevates interferon production, and inhibits replication of temperature-sensitive pathogens. By blocking PGE₂ synthesis with antipyretics, we may reduce the efficacy of these defenses, potentially prolonging infection. Animal studies have shown increased mortality in febrile animals treated with antipyretics during certain infections. The argument for treating fever centers on patient comfort, reduced metabolic demand (each 1 °C rise increases basal metabolic rate by ~10–13%), and prevention of complications in vulnerable populations. For patients with limited cardiac or pulmonary reserve, the increased O₂ demand of fever may be dangerous. In pediatric patients, high fevers risk febrile seizures. A nuanced approach recognizes that in healthy adults with moderate fevers, allowing the fever to run its course may be beneficial, while antipyretics are justified when fever causes significant discomfort, dehydration risk, or in patients with cardiopulmonary disease, neurological vulnerability, or temperatures approaching 40 °C.

SUMMARY

Lesson Summary

Summary

The body's innate immune defenses operate through three interlocking systems. Inflammation is a localized vascular and cellular response initiated when sentinel cells detect PAMPs and DAMPs via pattern recognition receptors. Vasodilation, increased capillary permeability, and leukocyte extravasation (margination → diapedesis → chemotaxis) deliver neutrophils and plasma proteins to the site of injury, producing the cardinal signs of rubor, tumor, calor, dolor, and functio laesa. Fever is a systemic response in which endogenous pyrogens (IL-1, IL-6, TNF-α) drive PGE₂ synthesis in the hypothalamus, resetting the thermoregulatory set point upward to enhance immune function and inhibit pathogen growth.

The complement system comprises ~30 plasma proteins activated via three pathways—classical (antibody-mediated), lectin (mannose recognition), and alternative (spontaneous C3 hydrolysis)—all converging on C3 convertase. Key outcomes include opsonization (C3b), inflammation amplification (C3a/C5a anaphylatoxins), and direct pathogen lysis via the membrane attack complex (MAC). Host cells are protected by regulatory proteins (CD55, CD59). Together, these three innate mechanisms contain and eliminate threats within hours, while simultaneously activating adaptive immunity for long-term protection.