18.1 Immunoglobulin Response and Antigen-Antibody Methods

18.1 Immunoglobulin Response and Antigen-Antibody Methods

Quick Answer: IgM is first, does not cross the placenta, and is the usual acute marker—and the usual false-positive trap. IgG is later, long-lived, and transplacental; a four-fold titer rise between acute and convalescent sera documents current infection. IgA is the mucosal isotype. Methods on outline III.E are latex agglutination, EIA, chemiluminescence, immunofluorescence, and treponemal versus nontreponemal (RPR) tests. Sensitivity is not specificity. Hook effect and rheumatoid factor produce false results. Bacterial latex on CSF does not replace Gram stain or culture for acute meningitis.

Official outline III.E Serology (content guideline revised 2025-09-25) sits inside Domain III (Mycobacteriology, Virology, Parasitology, and Mycology; 20–30% of M). This section is the immunoassay engine: which immunoglobulin means “now” versus “ever,” how lattice and labeled assays work, and why a numeric EIA index is not a diagnosis. Viral marker panels are 18.2. Treponema, Borrelia, and IGRA are 18.3. III.E.4 encephalitis serology (West Nile virus and other arboviruses) is SM-only and is not M-tested.

Immunoglobulin response: IgM, IgG, IgA

Infection drives a stereotyped B-cell response. IgM is produced first. It is a pentamer with ten antigen-binding sites, excellent at agglutination and complement activation, and it does not cross the placenta. Pathogen-specific IgM usually becomes detectable within days to about two weeks after symptom onset, depending on the agent and the assay. Because it is the early isotype, IgM is the classic acute-phase marker. The same properties create traps. IgM assays have more false positives than IgG assays. IgM can persist for months after some infections. Rheumatoid factor (RF)—typically an IgM autoantibody against the Fc of IgG—can glue IgG already bound to antigen and look like pathogen-specific IgM. Polyclonal B-cell stimulation in EBV and other acute viral illnesses does the same.

IgG follows class switching. It is a monomer, the dominant serum isotype of immunologic memory, and it crosses the placenta. A single IgG result documents past infection or vaccination; it does not time the infection. Timing requires IgM (with its limitations), IgG avidity (especially CMV, 18.2), or paired titers.

IgA is the mucosal antibody (secretory IgA in saliva, respiratory secretions, and gut) and is also present in serum. It matters for mucosal pathogens and some congenital-infection workups, but M items more often hinge on IgM versus IgG than on IgA.

The serologic window is the interval after infection when the organism is present but antibody is still below the assay cutoff. HIV, HCV, and HBV all have windows; antigen or NAAT closes them (18.2). A negative antibody test in the window is not “the patient was never infected.”

Acute versus convalescent serology is how laboratories prove current infection when a single IgM is unreliable. Draw acute serum as early as possible. Draw convalescent serum about 10–14 days later (sometimes 2–4 weeks; follow the protocol in the stem). Test both specimens together, same method, preferably the same run. A four-fold rise in IgG titer (for example 1:8 to 1:32) is a two-tube difference in doubling dilutions and is accepted as diagnostic of current or very recent infection. A four-fold fall after therapy is the syphilis nontreponemal rule in 18.3. A single high titer is only as good as a validated cutoff; the exam prefers the paired four-fold rule.

Neonatal interpretation follows the placenta rule. IgG in a newborn may be maternal. Pathogen-specific IgM in the neonate is fetal and supports congenital infection. Do not call congenital rubella, CMV, or syphilis on neonatal IgG alone.

Antigen-antibody principles

Antibody binds epitope with affinity. Visible assays require lattice formation (agglutination, precipitation, flocculation) or a labeled detector (enzyme, fluorophore, acridinium ester). Precipitation and agglutination are maximal at the zone of equivalence, where antigen and antibody are in balanced proportion.

• Prozone: antibody excess. Lattices stay small; a strongly positive serum looks negative until it is diluted. Secondary syphilis RPR is the classic microbiology example.
• Postzone: antigen excess. The same false-negative lattice problem from the other side.
• Hook effect (high-dose hook): in sandwich immunoassays, enormous antigen occupies capture antibody and labeled antibody as separate complexes, so the sandwich never forms. The result is falsely low or negative. Dilute the specimen and the value rises. Hook is tested for analytes that can reach extreme concentrations, including cryptococcal antigen and some high-burden viral antigens.

Sensitivity is the true-positive rate among the infected (true positives / [true positives + false negatives]). A highly sensitive test, when negative, helps rule out disease. Specificity is the true-negative rate among the uninfected. A highly specific test, when positive, helps rule in. Screening assays are often built for sensitivity; confirmatory assays for specificity. Positive and negative predictive values then move with prevalence: the same HIV EIA has a lower PPV in a low-prevalence prenatal panel than in a high-prevalence clinic.

Rheumatoid factor is the named interfering antibody. RF is usually IgM against IgG Fc. In an indirect IgM EIA, RF binds IgG that has already bound antigen and produces a false-positive IgM. Heterophile antibodies do analogous damage. Many IgM assays pretreat with anti-RF reagent or IgG absorbent; if the stem mentions RF-positive serum and a surprising IgM, think interference, not a new pathogen. Helicobacter pylori IgG is a related microbiology pairing from Chapter 8: serology documents exposure, not active gastric infection, which is why urea breath test and stool antigen exist.

Testing methods named on the outline

Latex agglutination coats polystyrene beads with antigen (to detect antibody) or with antibody (to detect antigen). Visible clumping is a positive. It is fast and requires no instrument. Historical CSF bacterial antigen latex targeted S. pneumoniae, H. influenzae type b, N. meningitidis, and sometimes GBS. Sensitivity is modest, especially after antibiotics, and specificity is imperfect. Do not serology-only diagnose acute bacterial meningitis. Pair every CSF antigen idea with cytospin Gram stain, culture, and, when indicated, multiplex PCR (Chapter 6). Cryptococcal antigen latex or EIA on serum or CSF remains useful because fungal burden is often high and culture is slow—that is a different organism and a different performance profile, not a license to skip the bacterial smear.

Enzyme immunoassay (EIA/ELISA) immobilizes antigen or antibody on a solid phase, adds specimen, then an enzyme-conjugated detector and substrate. Color (or fluorescence) is proportional to bound analyte. Indirect EIA detects antibody; sandwich EIA detects antigen; IgM-capture (μ-capture) reduces RF interference. EIA is the workhorse screen for hepatitis, HIV, EBV, and Lyme.

Chemiluminescence immunoassay (CLIA/CIA) replaces color with a light-emitting label (acridinium, luminol, alkaline phosphatase–dioxetane). Automated random-access analyzers use it for high-throughput serology. Analytical sensitivity is typically higher than older colorimetric EIA. Reverse-sequence syphilis screening is often a treponemal CIA (18.3). Light intensity is not a clinical titer unless the assay is built as a quantitative index.

Immunofluorescence splits by what is labeled. DFA applies fluorescent antibody directly to antigen in cells on a slide (viral antigen detection, not host serology). IFA applies patient serum to antigen spots, then fluorescent anti-human IgG or IgM. FTA-ABS is a treponemal IFA. Read fluorescence against controls; the endpoint titer is the last dilution that still fluoresces.

Treponemal versus nontreponemal (RPR) are the syphilis pair the outline names in the methods list. Nontreponemal tests (RPR, VDRL) detect reagin antibody against cardiolipin–lecithin–cholesterol antigen. They are flocculation assays, reported as reactive with a titer, and they fall with treatment. Treponemal tests (EIA, CIA, FTA-ABS, TP-PA) use T. pallidum antigens and usually remain positive for life. Method principle lives here; stages and algorithms live in 18.3.

Pair serology with microbiology

Serology answers “has the host responded?” Direct detection answers “is the organism here now?” Acute bacterial meningitis is a minutes-to-hours problem of stain, adjunct antigen, NAAT, and culture. A negative CSF latex does not exclude pneumococcus. A positive HIV antibody does not measure plasma RNA. An IGRA does not replace AFB smear for active TB. When an item mixes a febrile infant, cloudy CSF, and a latex kit, the M answer still starts with Gram stain and culture, not with serology alone.