Antigen-Antibody Interactions and Complement

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:

K_a = [AgAb] / ([Ag] × [Ab])

A larger K_a means stronger binding under the stated conditions. K_d, the dissociation constant, has the inverse relationship: a smaller K_d 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 is the epitope recognized by an antibody. 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.

Lattice formation and reaction zones

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:

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. An unexpectedly low result, unusual reaction kinetics, or disagreement with other findings can prompt dilution studies 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 are most useful when the same method and interpretation rules are used.

In double immunodiffusion, fused precipitin lines indicate identity between the recognized determinants. Crossing lines indicate nonidentity, while a fused line with a spur indicates partial identity. Many current clinical assays use other formats, but the same zone and cross-linking principles explain falsely weak reactions and the need for method-specific controls.^1,2,3

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. Addition of C3b produces the C5 convertase C4b2aC3b. ⁴

Lectin pathway

Mannose-binding lectin and ficolins bind selected carbohydrate or acetylated patterns on a surface. Their associated proteases, mainly 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.

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

The affected component determines the usual laboratory and infection pattern of a complement deficiency. Expression still varies with the specific variant, residual function, age, vaccination, and other immune defenses.⁶

Detailed immunodeficiency evaluation is covered in the next Immunology module. ⁶

Laboratory assessment

Complement concentration, pathway function, and in vivo activation are separate questions. They require different methods and may require different specimens.⁷

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 points toward 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. Normal CH50 and AH50 do not exclude a lectin-pathway defect.^7,9

Functional complement is labile, so collection and transport must follow the performing laboratory’s instructions. Current Mayo Clinic Laboratories instructions for CH50 and AH50 call for serum kept on wet ice, separated promptly, and frozen soon after centrifugation. 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 while generating fragments. This can lower functional results and raise activation-product results.^7,8,9

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 rests on the complete pattern and confirmation 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. Correct interpretation requires the agent, target, dose timing, assay method, and the laboratory’s 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 require an agent-specific interpretation from a validated assay.^7,9

References

1. Abbas AK, Lichtman AH, Pillai S, Henrickson S. Cellular and Molecular Immunology. 11th ed. Elsevier; 2025. Elsevier.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. Mayo Clinic Laboratories. Complement, total, serum. Test ID COM. Accessed August 29, 2026.
9. Mayo Clinic Laboratories. Alternative complement pathway, functional, serum. Test ID AH50. Accessed August 29, 2026.
10. Mayo Clinic Laboratories. C1 esterase inhibitor, functional, serum. Test ID C1INF. Accessed August 29, 2026.