Foundations of Innate and Adaptive Immunity

Innate Immunity and Adaptive Immunity

Innate Immunity

Innate immunity is the rapid, germline-encoded defense system present before exposure to a pathogen. It responds within minutes to hours, does not require prior sensitization, and recognizes conserved microbial or injury-associated patterns rather than unique antigenic epitopes. Its major functions are to prevent microbial entry, contain early infection, activate inflammation, and instruct the later adaptive response.

The innate immune system recognizes two broad categories of molecular signals. Pathogen-associated molecular patterns (PAMPs) are conserved microbial structures, such as lipopolysaccharide (LPS) in gram-negative bacteria, peptidoglycan, flagellin, unmethylated CpG DNA, and viral double-stranded RNA. Damage-associated molecular patterns (DAMPs) are host-derived molecules released from injured or necrotic cells, such as ATP, uric acid crystals, heat shock proteins, and extracellular DNA. These signals are detected by pattern recognition receptors (PRRs), including Toll-like receptors (TLRs), NOD-like receptors, RIG-I-like receptors, and cytosolic DNA sensors.

Toll-like receptors are especially high-yield. TLR4 recognizes LPS; TLR3 recognizes double-stranded RNA; TLR7 and TLR8 recognize single-stranded RNA; TLR9 recognizes unmethylated CpG DNA. PRR activation induces transcription factors such as NF-κB and IRF3/IRF7, leading to cytokine production. NF-κB promotes inflammatory cytokines including TNF-α, IL-1, and IL-6, while IRF pathways promote type I interferons such as IFN-α and IFN-β, which establish an antiviral state.

Innate Component	Primary Function	High-Yield Associations
Physical barriers	Prevent microbial entry	Skin keratin, tight junctions, mucus, cilia, gastric acid with pH approximately 1.5-3.5
Neutrophils	Acute bacterial and fungal killing	Most abundant leukocyte; normal adult absolute neutrophil count approximately 1,500-8,000/µL
Macrophages	Phagocytosis, cytokine secretion, antigen presentation	Derived from monocytes or tissue-resident precursors; produce TNF-α, IL-1, IL-6, IL-12
Dendritic cells	Link innate and adaptive immunity	Most important antigen-presenting cells for naive T-cell activation
Natural killer cells	Kill virally infected and tumor cells	Recognize decreased MHC class I; kill via perforin and granzymes
Complement	Opsonization, inflammation, lysis	C3b opsonizes; C3a/C5a are anaphylatoxins; C5a is strongly chemotactic

Innate inflammation follows a stereotyped vascular and cellular sequence. Histamine and nitric oxide cause vasodilation, producing redness and warmth. Increased vascular permeability allows plasma proteins to enter tissue, causing edema. Neutrophils adhere to endothelium through selectins and integrins, then migrate toward chemoattractants such as IL-8, C5a, leukotriene B4, and bacterial N-formylmethionine peptides. Phagocytes kill microbes using reactive oxygen species, nitric oxide, lysosomal enzymes, and antimicrobial peptides. Defects in these mechanisms produce classic Step 1 syndromes: chronic granulomatous disease from impaired NADPH oxidase, leukocyte adhesion deficiency from impaired neutrophil extravasation, and Chediak-Higashi syndrome from impaired phagolysosome fusion.

Adaptive Immunity

Adaptive immunity is the antigen-specific immune response mediated by lymphocytes. It develops more slowly than innate immunity, typically requiring 3-7 days after first exposure, but it has three defining properties: specificity, diversity, and memory. Specificity means that lymphocyte receptors recognize particular antigens. Diversity means that the immune system can recognize an enormous range of antigens; the theoretical receptor repertoire is often estimated at greater than 10¹¹ unique specificities. Memory means that re-exposure produces a faster, larger, and more effective response.

Adaptive immunity is divided into humoral immunity and cell-mediated immunity. Humoral immunity is mediated by B lymphocytes and antibodies, which are most effective against extracellular microbes and toxins. Antibodies neutralize pathogens, opsonize microbes for phagocytosis, activate complement, and mediate antibody-dependent cellular cytotoxicity. Cell-mediated immunity is mediated by T lymphocytes. CD4⁺ helper T cells coordinate immune responses through cytokine secretion, while CD8⁺ cytotoxic T cells kill infected or malignant cells, especially those harboring intracellular pathogens such as viruses.

Feature	Innate Immunity	Adaptive Immunity
Timing	Immediate; minutes to hours	Delayed primary response; usually 3-7 days
Recognition	Germline-encoded PRRs recognize PAMPs and DAMPs	Somatically rearranged B-cell and T-cell receptors recognize specific antigens
Specificity	Limited; recognizes conserved patterns	Highly specific; recognizes discrete epitopes
Memory	Classically absent or limited	Robust memory; basis of vaccination
Major cells	Neutrophils, macrophages, dendritic cells, NK cells, mast cells	B cells, plasma cells, CD4+ T cells, CD8+ T cells

The molecular basis of adaptive diversity is somatic recombination of antigen receptor genes. B-cell receptors and T-cell receptors are generated by V(D)J recombination mediated by RAG1 and RAG2. This process is random, so developing lymphocytes must undergo selection to prevent dangerous self-reactivity. Failure of tolerance can lead to autoimmune disease, while failure of lymphocyte development can cause severe immunodeficiency.

Innate and adaptive immunity are not separate silos. Dendritic cells capture antigen in peripheral tissues, become activated by PRR signaling, migrate to lymph nodes, and present antigen to naive T cells. Innate cytokines influence adaptive differentiation; for example, IL-12 promotes Th1 responses, whereas IL-4 promotes Th2 responses. Conversely, adaptive responses amplify innate effector mechanisms: antibodies enhance phagocytosis via Fc receptors and complement, while IFN-γ from Th1 cells activates macrophages. For Step 1, the key conceptual framework is that innate immunity recognizes patterns rapidly, whereas adaptive immunity recognizes specific antigens with memory.

T-Cell and B-Cell Development and Activation

T-Cell Development

T lymphocytes arise from hematopoietic stem cells in the bone marrow but complete maturation in the thymus. Their central task is to generate a diverse T-cell receptor (TCR) repertoire while eliminating cells that either cannot recognize self-MHC or recognize self-antigen too strongly. The mature naïve T cell is therefore MHC-restricted and self-tolerant.

Early thymocytes lack both CD4 and CD8 and are called double-negative cells. They rearrange TCR genes using RAG1/RAG2 recombinase, which mediates V(D)J recombination according to the 12/23 rule. Most T cells become αβ T cells, whose TCR consists of an α chain and β chain associated with CD3, the signaling complex. A minority become γδ T cells, which are enriched at epithelial surfaces and recognize antigen less dependently on classical MHC.

Stage	Phenotype	Key event	High-yield association
Double-negative	CD4− CD8−	TCR β-chain rearrangement	RAG defects cause severe combined immunodeficiency
Double-positive	CD4+ CD8+	Positive and negative selection	Most thymocytes die by apoptosis
Single-positive	CD4+ or CD8+	Lineage commitment	CD4 recognizes MHC II; CD8 recognizes MHC I

Positive selection occurs in the thymic cortex. Cortical thymic epithelial cells present self-peptide on MHC I and MHC II. T cells that bind self-MHC weakly to moderately survive; cells that fail to bind die by “death by neglect.” This step enforces MHC restriction. Recognition of MHC II promotes CD4 lineage commitment, whereas recognition of MHC I promotes CD8 lineage commitment.

Negative selection occurs mainly in the thymic medulla. Medullary thymic epithelial cells and dendritic cells present self-antigens; thymocytes with high-affinity binding undergo apoptosis. The AIRE gene allows thymic expression of peripheral tissue antigens, such as insulin and thyroid proteins. AIRE mutation causes autoimmune polyendocrine syndrome type 1, classically with chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenal insufficiency. Approximately 95% of developing thymocytes die during selection, emphasizing the stringency of central tolerance.

T-Cell Activation

Naïve T-cell activation occurs in secondary lymphoid organs and requires coordinated signals. Signal 1 is antigen-specific: the TCR-CD3 complex recognizes peptide-MHC on an antigen-presenting cell. CD4 or CD8 acts as a coreceptor and binds the nonpolymorphic region of MHC II or MHC I, respectively. Signal 2 is costimulation: CD28 on T cells binds B7-1/B7-2 on antigen-presenting cells. Without costimulation, the T cell becomes anergic, a mechanism of peripheral tolerance. Signal 3 consists of cytokines that drive differentiation.

After activation, intracellular calcium activates calcineurin, which dephosphorylates NFAT, allowing transcription of IL-2. IL-2 promotes clonal expansion through the high-affinity IL-2 receptor containing CD25. Clinically, calcineurin inhibitors exploit this pathway: cyclosporine binds cyclophilin and tacrolimus binds FKBP; both inhibit calcineurin and decrease IL-2 transcription. Abatacept, a CTLA-4-Ig fusion protein, binds B7 and blocks CD28-mediated costimulation.

T-cell subset	Key cytokine driver	Signature cytokines/functions	Classic target
Th1	IL-12	IFN-γ; activates macrophages and supports cytotoxic T cells	Intracellular microbes
Th2	IL-4	IL-4, IL-5, IL-13; eosinophils, IgE class switching, mucus	Helminths and allergy
Th17	IL-6, TGF-β, IL-23	IL-17, IL-22; neutrophil recruitment	Extracellular bacteria and fungi
Treg	IL-2, TGF-β	IL-10, TGF-β; suppresses immune responses	Peripheral tolerance

B-Cell Development

B lymphocytes develop in the bone marrow and mature in secondary lymphoid follicles. Like T cells, they generate antigen receptor diversity through RAG-mediated V(D)J recombination, but the B-cell receptor is membrane-bound immunoglobulin rather than a TCR. Heavy-chain rearrangement occurs first, followed by light-chain rearrangement. Successful immature B cells express surface IgM; mature naïve B cells coexpress IgM and IgD by alternative RNA splicing.

Central tolerance in the bone marrow removes or edits autoreactive B cells. Strong self-antigen binding may induce apoptosis, anergy, or receptor editing, in which additional light-chain rearrangement changes specificity. Failure of B-cell development at the pre-B-cell stage occurs in Bruton tyrosine kinase deficiency, causing X-linked agammaglobulinemia with absent germinal centers, very low immunoglobulins of all classes, and recurrent bacterial and enteroviral infections after maternal IgG wanes at about 6 months of age.

B-Cell Activation

B cells can be activated by T-dependent or T-independent antigens. T-dependent activation usually involves protein antigens. The BCR binds native antigen, internalizes it, and presents peptide on MHC II to a previously activated CD4+ T follicular helper cell. The essential costimulatory interaction is CD40 on B cells binding CD40L on T cells. CD40L deficiency causes X-linked hyper-IgM syndrome: defective class switching and germinal center formation, with high or normal IgM and low IgG, IgA, and IgE.

Within germinal centers, activated B cells undergo somatic hypermutation and class-switch recombination, both requiring activation-induced cytidine deaminase. Somatic hypermutation creates point mutations in immunoglobulin variable regions, followed by selection of higher-affinity clones, termed affinity maturation. Class switching changes the heavy-chain constant region while preserving antigen specificity, allowing production of IgG, IgA, or IgE. T-independent activation, typically by polysaccharides or lipopolysaccharide, produces mainly low-affinity IgM with limited memory; this explains poor responses to polysaccharide antigens in children younger than 2 years.

MHC Class I and II

MHC Class I

Major histocompatibility complex (MHC) class I molecules present endogenous intracellular peptides to CD8+ cytotoxic T lymphocytes. In humans, MHC molecules are called human leukocyte antigens (HLA) and are encoded on chromosome 6p21. The main class I loci are HLA-A, HLA-B, and HLA-C. MHC genes are highly polymorphic and are expressed codominantly, meaning both maternal and paternal alleles are expressed; this maximizes the range of peptides that can be presented.

MHC class I is expressed on all nucleated cells; mature erythrocytes lack nuclei and do not express MHC I. This distribution allows CD8+ T cells to survey most body cells for intracellular infection or malignant transformation. The MHC I molecule consists of a large α chain with 3 extracellular domains and a noncovalently associated β2-microglobulin subunit. The peptide-binding groove is formed by the α1 and α2 domains and is closed-ended, so it typically binds short peptides of approximately 8–10 amino acids.

The class I antigen-processing pathway begins with cytosolic proteins, such as viral proteins or tumor neoantigens, being degraded by the proteasome. Peptide fragments are transported from the cytosol into the rough endoplasmic reticulum by TAP1/TAP2 transporters, which are ATP-dependent peptide transporters. Within the endoplasmic reticulum, peptides are loaded onto newly synthesized MHC I molecules with the help of chaperone proteins. The MHC I–peptide complex then travels through the Golgi apparatus to the cell surface, where it is recognized by the T-cell receptor (TCR) of CD8+ T cells. The CD8 coreceptor binds the α3 domain of MHC I.

A key principle is MHC restriction: a T cell recognizes antigen only when the peptide is displayed by self-MHC. For example, a CD8+ T cell specific for a viral peptide will respond only if that peptide is presented on the appropriate self-HLA class I molecule. Cells with reduced MHC I expression, a common viral or tumor immune-evasion strategy, may escape CD8+ T-cell recognition but become more susceptible to natural killer (NK) cell killing because NK cells detect “missing self.”

MHC Class II

MHC class II molecules present exogenous extracellular peptides to CD4+ helper T lymphocytes. The principal class II loci are HLA-DP, HLA-DQ, and HLA-DR, also encoded within the HLA region on chromosome 6. In contrast to MHC I, MHC II expression is limited mainly to professional antigen-presenting cells (APCs): dendritic cells, macrophages, and B cells. Thymic epithelial cells also express MHC II for T-cell selection. Expression is upregulated by interferon-γ (IFN-γ), a cytokine classically produced by Th1 cells and NK cells.

MHC II consists of an α chain and a β chain, both transmembrane glycoproteins. Its peptide-binding groove is formed by the α1 and β1 domains and is open-ended, allowing binding of longer peptides, typically approximately 13–18 amino acids or longer. The CD4 coreceptor binds the β2 domain of MHC II.

The class II processing pathway begins when extracellular proteins are internalized by endocytosis or phagocytosis into acidified vesicles. Meanwhile, newly synthesized MHC II molecules in the endoplasmic reticulum are bound by the invariant chain, which blocks the peptide-binding groove and prevents inappropriate loading of endogenous ER peptides. The invariant chain also directs MHC II into endosomal compartments. Proteolysis degrades the invariant chain, leaving CLIP in the groove. HLA-DM facilitates removal of CLIP and loading of antigenic peptide. The peptide–MHC II complex then traffics to the surface to activate CD4+ T cells.

Feature	MHC Class I	MHC Class II
Main HLA loci	HLA-A, HLA-B, HLA-C	HLA-DP, HLA-DQ, HLA-DR
Expression	All nucleated cells	Professional APCs: dendritic cells, macrophages, B cells
Peptide source	Endogenous cytosolic proteins	Exogenous extracellular proteins
Peptide length	8–10 amino acids	13–18 amino acids or longer
T-cell interaction	CD8+ cytotoxic T cells	CD4+ helper T cells
Key processing proteins	Proteasome, TAP1/TAP2	Invariant chain, CLIP, HLA-DM

Clinically and for Step 1, MHC associations are high-yield because certain HLA alleles strongly increase autoimmune disease risk. Classic examples include HLA-B27 with ankylosing spondylitis and other seronegative spondyloarthropathies; HLA-DQ2/DQ8 with celiac disease; HLA-DR3/DR4 with type 1 diabetes mellitus; and HLA-DR2 with multiple sclerosis and Goodpasture syndrome. These associations reflect altered peptide presentation and loss of immune tolerance, not direct causation by the HLA molecule alone.

Defects in MHC expression cause characteristic immunodeficiencies. Bare lymphocyte syndrome type I, often due to defective TAP transport, reduces MHC I expression and impairs CD8+ T-cell function. Bare lymphocyte syndrome type II results from defective MHC II transcription, causing impaired CD4+ T-cell development and activation, severe recurrent infections, and low CD4+ T-cell counts. In transplantation, HLA matching is crucial because T cells can strongly recognize nonself MHC molecules; this alloreactivity underlies graft rejection and graft-versus-host disease.

Immunoglobulin Structure and Function

Core Antibody Architecture

Immunoglobulins, or antibodies, are soluble or membrane-bound antigen receptors produced by B cells and plasma cells. The basic immunoglobulin monomer is a symmetric Y-shaped glycoprotein composed of 2 identical heavy chains and 2 identical light chains linked by disulfide bonds. Light chains are either κ or λ; a single antibody molecule contains one type only. In normal serum, the κ:λ ratio is approximately 2:1, a high-yield clue because monoclonal plasma cell disorders often show marked skewing.

Each chain has a variable region and a constant region. The variable regions of one heavy and one light chain form the antigen-binding site. Within the variable region are complementarity-determining regions, which physically contact antigen and determine specificity. The constant region of the heavy chain determines the antibody isotype: IgM, IgD, IgG, IgA, or IgE.

• Fab fragment: “fragment antigen-binding”; contains variable regions and binds antigen.
• Fc fragment: “fragment crystallizable”; binds Fc receptors on immune cells and activates effector functions such as complement fixation, placental transfer, opsonization, and mast cell activation.
• Hinge region: provides flexibility, allowing antibodies to bind antigens separated by variable distances.

Isotypes and High-Yield Functions

Isotype	Structure and Location	Major Functions	Step 1 Associations
IgM	Pentamer in serum; monomer on naive B cells	First antibody produced in primary response; strongest activator of classical complement	Indicates recent infection; excellent agglutinin due to 10 antigen-binding sites
IgD	Monomer on naive B cells	B-cell antigen receptor with IgM	Low serum concentration; mainly developmental marker
IgG	Monomer; most abundant serum immunoglobulin	Opsonization, neutralization, complement activation, antibody-dependent cellular cytotoxicity, placental transfer	Only isotype that crosses placenta via neonatal Fc receptor; half-life about 21 days
IgA	Monomer in serum; dimer in secretions with J chain and secretory component	Mucosal immunity in saliva, tears, breast milk, GI and respiratory secretions	Protects mucosa without strong inflammation; breast milk IgA protects neonatal gut
IgE	Monomer bound to FcεRI on mast cells and basophils	Immediate hypersensitivity; defense against helminths via eosinophils	Cross-linking triggers mast cell degranulation; shortest serum half-life, about 2 days

Approximate adult serum concentrations are: IgG 700–1600 mg/dL, IgA 70–400 mg/dL, IgM 40–230 mg/dL, with IgE normally present at very low concentrations, commonly <100–150 IU/mL depending on laboratory reference range.

Antibody Effector Mechanisms

Antibodies do not usually kill pathogens directly; they mark, neutralize, or recruit other immune mechanisms. Neutralization occurs when antibodies block microbial adherence, toxin binding, or viral entry. Opsonization occurs when IgG coats pathogens and binds Fcγ receptors on macrophages and neutrophils, increasing phagocytosis. Classical complement activation is triggered primarily by IgM and IgG binding antigen; IgM is especially efficient because one pentamer can bind C1q, whereas IgG generally requires adjacent Fc regions. Complement promotes C3b-mediated opsonization, inflammation through C3a and C5a, and lysis via the C5b-9 membrane attack complex.

Antibody-dependent cellular cytotoxicity occurs when NK cells bind IgG-coated target cells through FcγRIII, also called CD16, and induce apoptosis using perforin and granzymes. IgE mediates allergic and antiparasitic responses: antigen cross-linking of mast cell-bound IgE causes degranulation with histamine release, while eosinophils bind IgE-coated helminths and release major basic protein.

Diversity, Class Switching, and Affinity Maturation

Antibody specificity begins with V(D)J recombination in developing B cells. Heavy chains rearrange V, D, and J segments; light chains rearrange V and J segments. This process uses RAG1/RAG2 and creates junctional diversity. After antigen exposure, activated B cells in germinal centers undergo somatic hypermutation and affinity maturation, processes mediated by activation-induced cytidine deaminase. B cells with receptors that bind antigen most strongly receive survival signals from follicular helper T cells and become high-affinity plasma cells or memory B cells.

Class-switch recombination changes the heavy-chain constant region while preserving antigen specificity. Thus, a B cell can switch from IgM to IgG, IgA, or IgE without changing what antigen it recognizes. This also requires activation-induced cytidine deaminase. Cytokines influence isotype choice: IL-4 promotes IgE class switching, TGF-β promotes IgA, and IFN-γ favors opsonizing IgG subclasses. Defective CD40L on helper T cells prevents class switching, producing hyper-IgM syndrome with high or normal IgM and low IgG, IgA, and IgE.

Clinical and Exam-Relevant Applications

In a primary immune response, IgM appears first, followed by class-switched IgG. In a secondary response, memory B cells generate a faster, larger, higher-affinity IgG-predominant response. This principle explains vaccine boosters and serologic interpretation: pathogen-specific IgM suggests recent infection, whereas IgG suggests prior exposure, vaccination, or later-stage infection.

Therapeutic pooled immunoglobulin illustrates antibody function clinically. Intravenous immunoglobulin is pooled IgG from many donors and is used for antibody replacement and immunomodulation; typical replacement dosing is approximately 400–600 mg/kg every 3–4 weeks, while immunomodulatory dosing is commonly 2 g/kg total divided over 2–5 days. For Step 1, the key concept is passive immunity: transferred antibodies provide immediate protection but do not generate immunologic memory.

Cytokines

Cytokines are small, soluble proteins secreted by immune and nonimmune cells that mediate communication between cells. They function at very low concentrations, typically in the picomolar to nanomolar range, and act through high-affinity cell-surface receptors. Cytokines may act in an autocrine manner on the same cell that produced them, a paracrine manner on nearby cells, or an endocrine manner systemically. They are central to both innate and adaptive immunity: innate immune cells use cytokines to initiate inflammation and antiviral defense, whereas adaptive immune cells use cytokines to direct lymphocyte proliferation, differentiation, and effector function.

General Principles of Cytokine Signaling

Cytokines are pleiotropic, meaning one cytokine can have multiple effects on different cell types; they are also redundant, meaning several cytokines may produce overlapping effects. This redundancy explains why blocking one cytokine may incompletely suppress an immune response. Many cytokine receptors signal through the JAK-STAT pathway: cytokine binding causes receptor dimerization, activation of Janus kinases, phosphorylation of STAT transcription factors, STAT dimerization, and nuclear translocation to regulate gene expression. Other cytokines signal through NF-κB and MAP kinase pathways, especially proinflammatory cytokines such as IL-1 and TNF-α.

High-Yield Cytokines by Function

Cytokine	Major Source	Major Function	Classic Step 1 Association
IL-1	Macrophages	Fever, endothelial activation, acute inflammation	Endogenous pyrogen; induces prostaglandin E2 in hypothalamus
IL-2	Activated T cells	T-cell proliferation; supports regulatory T cells	Driven by calcineurin/NFAT; inhibited by cyclosporine and tacrolimus
IL-3	T cells	Stimulates bone marrow hematopoietic progenitors	Broad hematopoietic growth factor
IL-4	Th2 cells, mast cells	Th2 differentiation; class switching to IgE	Allergy and helminth defense
IL-5	Th2 cells	Eosinophil growth/activation; class switching to IgA	Eosinophilia in parasitic infection and asthma
IL-6	Macrophages, T cells	Fever; acute-phase protein synthesis by liver	Increases CRP, fibrinogen, hepcidin; anemia of chronic disease
IL-8	Macrophages, endothelial cells	Neutrophil chemotaxis	Chemokine; recruits neutrophils to infection
IL-10	Regulatory T cells, macrophages	Inhibits macrophages and Th1 responses	Anti-inflammatory cytokine
IL-12	Macrophages, dendritic cells	Th1 differentiation; stimulates NK cells	Promotes IFN-γ production
IL-17	Th17 cells	Recruits neutrophils; promotes inflammation	Important against extracellular bacteria and fungi
TNF-α	Macrophages	Fever, cachexia, endothelial activation, septic shock	Maintains granulomas; anti-TNF therapy predisposes to TB reactivation
IFN-γ	Th1 cells, NK cells	Activates macrophages; increases MHC I and II expression	Defense against intracellular pathogens
IFN-α/β	Virus-infected cells	Antiviral state; increases MHC I expression	Inhibit viral replication via protein kinase R and RNase L pathways
TGF-β	Regulatory T cells, macrophages	Suppresses immune responses; promotes tissue repair and fibrosis	With IL-6 favors Th17 differentiation; alone favors Treg development

Cytokines and T-Helper Cell Differentiation

Naive CD4+ T cells differentiate into specialized T-helper subsets depending on the cytokine milieu. IL-12 promotes Th1 differentiation, which produces IFN-γ and activates macrophages for killing intracellular organisms such as mycobacteria. IL-4 promotes Th2 differentiation, which produces IL-4, IL-5, and IL-13 to support IgE production, eosinophil activation, mast cell responses, and helminth defense. IL-6 plus TGF-β promotes Th17 differentiation; Th17 cells secrete IL-17 and IL-22, strengthening epithelial barriers and recruiting neutrophils. TGF-β plus IL-2 supports regulatory T-cell development, which dampens immune activation through IL-10 and TGF-β.

Fever, Acute-Phase Response, and Sepsis

IL-1, IL-6, and TNF-α are the major endogenous pyrogens. They act on the hypothalamus to increase prostaglandin E2, raising the temperature set point; antipyretics such as NSAIDs reduce fever by inhibiting cyclooxygenase-mediated prostaglandin synthesis. IL-6 stimulates hepatocytes to produce acute-phase proteins, including C-reactive protein, serum amyloid A, fibrinogen, and hepcidin. A commonly used reference value for CRP is <10 mg/L, although interpretation depends on clinical context.

Excessive systemic cytokine release contributes to septic shock. TNF-α and IL-1 increase endothelial adhesion molecule expression, vascular permeability, and nitric oxide production, causing vasodilation and hypotension. IL-6 levels often correlate with inflammatory severity, but single cytokine measurements are not generally used as Step 1 diagnostic thresholds.

Pharmacologic Correlations

Target	Example Drug	Mechanism	High-Yield Toxicity/Association
TNF-α	Infliximab, adalimumab, etanercept	Neutralize TNF-α signaling	Reactivation of latent tuberculosis; screen with PPD or interferon-γ release assay before therapy
IL-6 receptor	Tocilizumab	Blocks IL-6-mediated inflammatory signaling	Used in rheumatoid arthritis and cytokine release syndrome
IL-1 receptor	Anakinra	IL-1 receptor antagonist	Used in autoinflammatory syndromes and rheumatoid arthritis
IL-12/23	Ustekinumab	Blocks p40 subunit shared by IL-12 and IL-23	Psoriasis and inflammatory bowel disease association
IL-17	Secukinumab	Neutralizes IL-17A	Can increase risk of mucocutaneous Candida infections

A key pharmacologic principle is that calcineurin inhibitors such as cyclosporine and tacrolimus reduce IL-2 transcription by blocking NFAT activation, thereby suppressing T-cell proliferation. This is distinct from cytokine-neutralizing biologics, which bind cytokines or cytokine receptors extracellularly.

Complement System

The complement system is a liver-derived, plasma-protein cascade that links innate and adaptive immunity. Most complement proteins circulate as inactive zymogens and are activated by proteolytic cleavage. Complement has three major effector functions: opsonization of microbes, inflammation via anaphylatoxins, and direct killing through the membrane attack complex (MAC).

Activation Pathways

All complement pathways converge on cleavage of C3 into C3a and C3b. C3 is the most abundant complement protein; typical adult serum C3 is approximately 90–180 mg/dL, and C4 is approximately 10–40 mg/dL, though ranges vary by laboratory.

Pathway	Trigger	C3 Convertase	High-Yield Association
Classical	IgM or IgG bound to antigen; also C-reactive protein	C4b2a	Links adaptive antibodies to innate effector killing
Lectin	Mannose-binding lectin binds microbial mannose residues	C4b2a	Antibody-independent but uses classical-pathway components C2 and C4
Alternative	Spontaneous C3 hydrolysis on microbial surfaces	C3bBb	Stabilized by properdin; continuously “ticks over” at low level

In the classical pathway, C1q binds the Fc region of antigen-bound IgM or IgG. IgM is especially efficient because one pentameric IgM molecule can activate C1, whereas IgG generally requires adjacent Fc regions. C1r and C1s then cleave C4 and C2 to form C4b2a, the classical C3 convertase. In the lectin pathway, mannose-binding lectin and MASP proteases generate the same C3 convertase. In the alternative pathway, C3b binds factor B, which is cleaved by factor D to form C3bBb.

Effector Molecules

• C3b: Major opsonin. It coats microbes and binds complement receptors, especially CR1 on phagocytes and erythrocytes. Erythrocyte CR1 transports immune complexes to the spleen and liver for clearance.
• C3a and C5a: Anaphylatoxins that promote mast-cell degranulation, increased vascular permeability, and smooth muscle contraction. C5a is the most potent chemotactic factor for neutrophils.
• C5b-C9: Forms the membrane attack complex. C5b recruits C6, C7, C8, and polymerized C9 to create pores in microbial membranes, especially effective against Neisseria.

After C3 convertase forms, addition of another C3b generates C5 convertase: classical/lectin C4b2a3b and alternative C3bBb3b. C5 convertase cleaves C5 into C5a and C5b, initiating MAC assembly.

Regulation and Host-Cell Protection

Complement must be tightly regulated because host cells are also vulnerable to membrane injury. Decay-accelerating factor (DAF/CD55) disrupts C3 convertases, and CD59 inhibits C9 polymerization, preventing MAC formation. These proteins are attached to membranes by GPI anchors. Loss of GPI-anchored CD55 and CD59, classically due to an acquired PIGA mutation, causes paroxysmal nocturnal hemoglobinuria, characterized by complement-mediated intravascular hemolysis, hemoglobinuria, pancytopenia, and thrombosis.

C1 esterase inhibitor inhibits C1r/C1s and kallikrein. Deficiency causes hereditary angioedema, with episodic nonpitting edema of skin, airway, or bowel due to excess bradykinin. Unlike histamine-mediated allergic angioedema, it typically lacks urticaria and responds poorly to antihistamines or epinephrine.

Complement Deficiencies

Deficiency	Clinical Pattern	Mechanism
C1q, C2, C4	SLE-like disease; recurrent immune-complex disease	Impaired immune-complex clearance and apoptotic debris removal
C3	Severe recurrent pyogenic infections; immune-complex disease	Loss of central opsonization and downstream complement activation
C5-C9	Recurrent Neisseria meningitidis or N. gonorrhoeae	Defective MAC formation
Factor D or properdin	Recurrent Neisseria infections	Impaired alternative pathway amplification
DAF/CD55 or CD59 loss	Paroxysmal nocturnal hemoglobinuria	Unregulated complement attack on RBC membranes
C1 esterase inhibitor	Hereditary angioedema	Excess bradykinin generation

Laboratory Testing and Pharmacology

CH50 assesses total classical complement function from C1 through C9. A low CH50 suggests deficiency or consumption in the classical or terminal pathway. AH50 assesses alternative pathway function. In immune-complex disease such as active systemic lupus erythematosus, complement is consumed, often producing low C3 and C4.

Complement-targeted drugs are high-yield mechanistically. Eculizumab and ravulizumab are monoclonal antibodies against C5, preventing C5a generation and MAC formation; they are used in paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome. Because terminal complement blockade mimics C5-C9 deficiency, patients are at increased risk for Neisseria infection and require meningococcal vaccination before therapy when feasible. C1 esterase inhibitor concentrate, icatibant bradykinin B2 receptor blockade, and kallikrein inhibition are mechanistically relevant therapies for hereditary angioedema.