The behavior of antigen, antibody, and complement explains two common bench findings: a serum that is nonreactive undiluted and reactive after dilution, and a low complement screen after delayed warm handling of the tube. Antigen and antibody bind through reversible molecular forces. The number, strength, and arrangement of the bonds determine whether the complexes stay soluble, form a visible lattice, recruit complement, or generate an assay signal.1
Binding strength and specificity
An antibody recognizes an epitope with its antigen-binding site. The matched surfaces are held together by hydrogen bonds, electrostatic forces, van der Waals forces, and hydrophobic interactions. Because each force is weak, stable binding depends on a close match in shape and charge.
Affinity is the strength of the bond between one antigen-binding site and one epitope. Its association constant at equilibrium is:
A larger Ka means stronger binding under the stated conditions. Kd, the dissociation constant, has the inverse relationship: a smaller Kd means stronger affinity.1,2
Avidity is the stability gained when a multivalent antibody forms several bonds with a multivalent antigen. IgM has ten theoretical antigen-binding sites, which can produce high avidity when several sites reach repeated epitopes. Actual avidity depends on epitope density, spacing, orientation, and access. IgG can also bind with substantial avidity when both Fab arms engage the same target.1,2
Specificity describes how selectively an antibody recognizes its target epitope. The epitope is the target structure. Cross-reactivity occurs when the same binding site recognizes a related structure. The related target often binds with lower affinity, but the result depends on the antibody and the structures involved. Reagent selection and assay validation determine which cross-reactions affect a reported result, so an unexpected reactive result is checked against the method's known cross-reactions before release.
Lattice formation and reaction zones
When a specimen is weak or nonreactive undiluted and reactive at higher dilutions, the cause is the proportion of antigen to antibody. A visible precipitate or agglutinate can form only when antibodies have at least two accessible binding sites and multivalent antigens permit repeated bridging. Precipitation produces insoluble immune complexes from soluble antigen. Agglutination joins cells, bacteria, or reagent beads into visible clumps. Antibody first coats, or sensitizes, each particle. Cross-links between the sensitized particles then form the visible lattice.
The proportions of antigen and antibody determine lattice size:
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| Reaction zone | Reactant balance | Complex formation | Laboratory consequence |
|---|---|---|---|
| Antibody excess, or prozone | Antibody occupies available epitopes while antigen is scarce | Small complexes form with too few antigen bridges | A reactive specimen can appear weak or nonreactive; testing validated specimen dilutions can restore the reaction |
| Equivalence | Available antigenic determinants and antibody sites support repeated cross-links | Large lattices form | Precipitation or agglutination is strongest |
| Antigen excess, or postzone | Antigen saturates available antibody sites | Small soluble complexes predominate | A high antigen concentration can produce a falsely low or nonreactive result; follow the method's dilution or high-dose protocol |
Sandwich immunoassays can also give a high-dose hook effect. Excess analyte prevents the expected capture-analyte-detector complex from forming, although the exact mechanism depends on the assay design. When a result is unexpectedly low, shows unusual reaction kinetics, or disagrees with other findings, repeat it at dilution under the validated procedure.3
An antibody titer is the reciprocal of the highest specimen dilution that reaches the method-defined reactive endpoint. If the final reactive dilution is 1-in-64 total, the reported titer is 64. The endpoint changes with the antigen, method, reaction grade, incubation conditions, and dilution convention. Serial titers can be compared only when the same method and interpretation rules are used, so run paired specimens by one method.
Reading paired titers. In an acute specimen, the twofold dilutions from 1:2 through 1:64 are reactive, and 1:128 and higher are nonreactive. The endpoint is 1:64, so the reported titer is 64. A convalescent specimen reaches 1:256 by the same method, and 256 ÷ 64 = 4, a fourfold rise. If the undiluted specimen or the lowest dilutions are nonreactive and later dilutions react, antibody excess in the prozone may have masked the reaction until dilution restored lattice formation.
In double immunodiffusion, fused precipitin lines indicate identity between the recognized determinants. Crossing lines indicate nonidentity. A fused line with a spur indicates partial identity. Many current clinical assays use other formats. The same zone and cross-linking principles explain their falsely weak reactions and their need for method-specific controls.1-3
Detailed solution calculations are covered in Dilution Math. Transfusion-specific binding, red-cell agglutination, and anti-human globulin testing are covered in Blood Group Immunology. General sensitivity, specificity, and predictive-value interpretation is covered in Preanalytic, Analytic, and Postanalytic Quality Assessment.
Complement activation
Complement includes soluble proteins, membrane proteins, regulators, and receptors. Its three activation pathways form enzyme complexes called convertases. Each pathway generates a C3 convertase, adds C3b to produce a C5 convertase, and can continue through the shared terminal pathway.4
Classical pathway
C1q recognizes Fc regions arranged on a surface, especially antigen-bound IgM or appropriately clustered IgG. This activates C1r and C1s. C1s cleaves C4 and C2, producing the classical C3 convertase C4b2a. The convertase cleaves C3 into C3a and C3b. Adding C3b produces the C5 convertase C4b2aC3b. 4
Lectin pathway
Mannose-binding lectin (MBL) and ficolins bind selected carbohydrate or acetylated patterns on a surface. Their associated proteases, mainly the MBL-associated serine proteases MASP-1 and MASP-2, support cleavage of C4 and C2. This produces the same C4b2a convertase as the classical pathway.4
Alternative pathway
Spontaneous low-level C3 hydrolysis produces C3(H2O), which binds factor B. Factor D cleaves the bound factor B and forms the fluid-phase C3(H2O)Bb convertase. C3b generated by this convertase can attach to a surface and bind another factor B. Factor D then cleaves factor B to form the surface convertase C3bBb, which properdin stabilizes. Factor H, factor I, and membrane regulators restrain this amplification on host cells. Another C3b molecule converts C3bBb into an alternative-pathway C5 convertase.
C3b deposited by the classical or lectin pathway can enter the same amplification loop. The balance between amplification and regulation determines how far the cascade proceeds.4
Terminal pathway
C5 convertases split C5 into C5a and C5b. C5b binds C6, C7, C8, and C9 to assemble the C5b-9 membrane attack complex (MAC). The MAC forms pores in susceptible membranes and is particularly important against susceptible gram-negative bacteria.4,5
Complement effects and regulation
Complement fragments promote inflammation, phagocytosis, B-cell signaling, and membrane injury.
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| Product | Principal effect | Important receptor or mechanism |
|---|---|---|
| C3a and C5a | Promote inflammation and vascular responses; C5a is also a strong leukocyte chemoattractant | C3aR and C5aR1 on responsive leukocytes and other cells |
| C3b and C4b | Coat targets and immune complexes for clearance | CR1, or CD35, binds both fragments |
| iC3b | Supports phagocyte adhesion and ingestion of coated targets | CR3, or CD11b-CD18, and CR4, or CD11c-CD18 |
| C3d and C3dg | Strengthen B-cell responses to complement-coated antigen | CR2, or CD21, acts with CD19 in the B-cell coreceptor complex |
| C5b-9 | Forms a membrane pore | C5b recruits C6 through C9 |
CR3 and CR4 both contain CD18. Leukocyte adhesion deficiency type 1 results from reduced or absent CD18 expression and impairs leukocyte adhesion, migration, and phagocytosis, including recognition of iC3b-coated targets.6
Soluble and membrane-bound regulators act at several points in the cascade. They protect host surfaces and limit complement activity to the appropriate surface and time.4,5
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| Regulator | Main action | Laboratory or disease connection |
|---|---|---|
| C1 inhibitor | Limits C1r, C1s, and lectin-pathway proteases; also regulates contact-system proteases | Reduced amount or function causes bradykinin-mediated hereditary angioedema |
| Factor H and factor I | Factor H accelerates alternative-convertase decay and acts as a factor I cofactor; factor I cleaves C3b and C4b with a cofactor | Loss of control can accompany atypical hemolytic uremic syndrome and C3 glomerulopathy |
| C4-binding protein and vitronectin | C4-binding protein supports factor I cleavage of C4b; vitronectin limits insertion of soluble C5b-7 into membranes | Limit classical and lectin convertases and fluid-phase terminal complexes |
| CR1, or CD35 | Provides factor I cofactor and convertase decay activity; binds complement-coated immune complexes | Protects host cells and supports immune-complex clearance |
| CD46, or membrane cofactor protein | Provides membrane-bound factor I cofactor activity for C3b and C4b | Protects host cells from complement amplification |
| CD55 and CD59 | CD55 accelerates convertase decay; CD59 blocks terminal-complex assembly | A clonal PIGA mutation in paroxysmal nocturnal hemoglobinuria removes these glycosylphosphatidylinositol-anchored proteins and increases complement injury |
The affected component determines the usual laboratory and infection pattern of a complement deficiency. Expression also varies with the specific variant, residual function, age, vaccination, and other immune defenses.6
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| Defect group | Main functional loss | Typical association |
|---|---|---|
| Early classical components, including C1, C2, and C4 | Classical activation and immune-complex clearance | Encapsulated bacterial infection and lupus-like autoimmunity |
| C3 | Central opsonization, amplification, and terminal-pathway recruitment | Severe recurrent infection with encapsulated bacteria and immune-complex disease |
| Alternative-pathway factors B, D, or properdin | Alternative activation or amplification | Invasive meningococcal disease and other pyogenic infection |
| Terminal components C5 through C9 | MAC formation | Recurrent invasive Neisseria infection |
| MBL or lectin-pathway components | Lectin-pathway recognition or activation | Variable expression; isolated MBL deficiency is often clinically silent |
Detailed immunodeficiency evaluation is covered in Diseases of the Immune System. 6
Laboratory assessment
Complement testing addresses three separate questions: how much of a component is present, whether a pathway completes the cascade, and whether complement is being activated in vivo. Each question needs its own method and may need its own specimen, so confirm which test is ordered before collection.7
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| Question | Test approach | Interpretation limit |
|---|---|---|
| How much component is present? | Nephelometry or immunoturbidimetry for routine C3 and C4; other antigenic immunoassays and method-specific radial immunodiffusion for selected components | Detects protein mass; a dysfunctional protein can still be present |
| Does a pathway complete the cascade? | CH50 for classical and shared terminal function; AH50 for alternative and shared terminal function; lectin-pathway functional assay when indicated | A low result can reflect deficiency, consumption, therapeutic blockade, or loss of activity during handling |
| Is complement being activated? | Method-specific activation products such as C3a, Ba, Bb, C4d, or soluble C5b-9 | Ex vivo activation can increase fragments after collection; the specimen and stabilizer must match the validated assay |
The traditional CH50 endpoint is the amount of serum that lyses 50% of antibody-sensitized erythrocytes. Current platforms may instead use sensitized liposomes and an enzyme signal. AH50 can be measured with erythrocytes under magnesium-EGTA conditions or by an enzyme immunoassay for terminal-complex formation. CH50 and AH50 name the pathways screened. The performing laboratory's method defines the specimen, units, reference interval, and analytical limits.8,9
CH50 and AH50 are usually interpreted together before individual components are measured. A low result in only one screen suggests a defect in a component unique to that pathway. Low activity in both screens can result from a shared C3 or terminal-pathway defect, active consumption, therapeutic blockade, or a preanalytic problem. CH50 and AH50 results within the reference interval do not exclude a lectin-pathway defect.7,9
Functional complement is labile, so follow the performing laboratory's collection and transport instructions exactly. Current Mayo Clinic Laboratories instructions for CH50 and AH50 are to place the whole-blood tube on wet ice immediately after collection and let it clot there, centrifuge it at 4 °C, aliquot the serum, and freeze the serum within 30 minutes of centrifugation. If a refrigerated centrifuge is unavailable, Mayo permits a room-temperature spin only when the tube remains on ice beforehand and the serum is aliquoted and frozen immediately afterward. Activation-product studies often require chilled EDTA plasma or another assay-specific inhibitor to limit activation after collection. Delayed warm handling can consume functional complement and generate fragments. Functional results can then read falsely low and activation-product results falsely high.7-9
Testing for suspected C1-inhibitor deficiency includes C4, C1-inhibitor antigen, and C1-inhibitor function. Type I hereditary angioedema usually lowers both antigen and function. Type II usually preserves or raises the antigen concentration but lowers function. C1q is usually preserved in hereditary C1-inhibitor deficiency and can be reduced in acquired deficiency. Classification depends on both the antigen and the function result, so report the complete pattern and confirm it with a correctly handled repeat specimen.10
Complement-directed therapy can intentionally lower a functional result. C5 blockade commonly suppresses both CH50 and AH50 because each screen depends on terminal-pathway completion. To interpret the result, the laboratory needs the agent, its target, the dose timing, the assay method, and its own validated monitoring criteria. Inhibitors of C3, factor B, or factor D can produce different patterns in upstream components and activation fragments. Soluble C5b-9 and other fragments also need an agent-specific interpretation from a validated assay. Before you release a low functional complement result, check how the specimen was handled and whether the patient receives complement-directed therapy.7,9
Practice
References
- Abbas AK, Lichtman AH, Pillai S, Henrickson S. Cellular and Molecular Immunology. 11th ed. Elsevier; 2025. Elsevier. Back to text
- Alberts B, Johnson A, Lewis J, et al. B cells and antibodies. In: Molecular Biology of the Cell. 4th ed. Garland Science; 2002. Accessed August 29, 2026. Back to text
- Jacobs JFM, van der Molen RG, Bossuyt X, Damoiseaux J. Antigen excess in modern immunoassays: to anticipate on the unexpected. Autoimmun Rev. 2015;14(2):160-167. doi:10.1016/j.autrev.2014.10.018 Back to text
- Merle NS, Church SE, Fremeaux-Bacchi V, Roumenina LT. Complement system part I: molecular mechanisms of activation and regulation. Front Immunol. 2015;6:262. doi:10.3389/fimmu.2015.00262 Back to text
- Merle NS, Noe R, Halbwachs-Mecarelli L, Fremeaux-Bacchi V, Roumenina LT. Complement system part II: role in immunity. Front Immunol. 2015;6:257. doi:10.3389/fimmu.2015.00257 Back to text
- Orange JS, Chinen J, Horner CC, et al. 2025 inborn errors of immunity practice parameter: guidance from the Joint Task Force on Practice Parameters, the American Academy of Allergy, Asthma & Immunology, the American College of Allergy, Asthma and Immunology, and the Clinical Immunology Society. Ann Allergy Asthma Immunol. 2026;136(4):426-493.e1. doi:10.1016/j.anai.2025.10.026 Back to text
- Willrich MAV, Braun KMP, Moyer AM, Jeffrey DH, Frazer-Abel A. Complement testing in the clinical laboratory. Crit Rev Clin Lab Sci. 2021;58(7):447-478. doi:10.1080/10408363.2021.1907297 Back to text
- Mayo Clinic Laboratories. Complement, total, serum. Test ID COM. Accessed August 29, 2026. Back to text
- Mayo Clinic Laboratories. Alternative complement pathway, functional, serum. Test ID AH50. Accessed August 29, 2026. Back to text
- Mayo Clinic Laboratories. C1 esterase inhibitor, functional, serum. Test ID C1INF. Accessed August 29, 2026. Back to text