🔬 Serology overview
foundation Serologic tests rely on the specific interaction between antibodies and antigens. These reactions occur both in vivo and in the laboratory, forming the basis for diagnosing infections, autoimmune disorders, and monitoring immune status.
Key clinical roles: detect pathogen exposure, measure vaccine response, diagnose autoimmune diseases, and monitor disease progression or treatment efficacy.
- Antibody (immunoglobulin): Y‑shaped glycoprotein produced by B cells; binds specific epitopes.
- Antigen: any molecule that can be recognized by antibodies or T‑cell receptors.
- Epitope: the precise region on an antigen recognized by an antibody.
🧬 IgM & IgG — primary vs secondary response
IgM
- First isotype produced during primary immune response.
- Pentameric structure; highly efficient at complement activation.
- clinical pearl IgM does not cross the placenta → detection in a neonate indicates in utero infection (e.g., rubella, CMV, toxoplasmosis).
- Serum IgM to hepatitis B core antigen (HBcAb) suggests acute or recent HBV infection; useful during the “window period.”
IgG
- Predominant isotype in secondary responses; high affinity.
- Crosses placenta → provides passive immunity to fetus.
- Four subclasses (IgG1–4) with different effector functions.
- Rising IgG titers (with or without IgM) indicate past infection or vaccination.
📌 High‑yield: In primary infection, IgM appears first, followed by IgG. A single high IgM titer is often diagnostic of acute infection; paired acute/convalescent IgG titers (4‑fold rise) confirm recent infection.
🧩 Idiotype · isotype · allotype
Idiotype – antigen‑binding region (variable domains). Unique to each B‑cell clone; millions of specificities. Anti‑idiotype antibodies can regulate immune responses.
Isotype – constant region of heavy chain (e.g., IgM, IgG, IgA). Determines effector function (complement fixation, opsonization, etc.).
Allotype – allelic variants of the same isotype among individuals. Minor differences in constant regions; can be immunogenic in transfused patients (e.g., type III hypersensitivity to pooled gamma globulin).
✂️ Antibody digestion: papain vs pepsin
- Papain cleaves above the hinge disulfide bonds → generates two Fab (antigen‑binding) fragments and one Fc (crystallizable) fragment.
- Pepsin cleaves below the hinge → produces a single F(ab')₂ fragment (divalent) and degrades the Fc portion.
- F(ab')₂ fragments are used in certain diagnostic assays to avoid Fc‑mediated interference.
IgG
⟶
Papain
⟶
2 Fab + Fc
Pepsin
⟶
F(ab')₂ + digested Fc
⚖️ Zone of equivalence & antigen‑antibody titration
Precipitation and agglutination are maximal when antigen and antibody are present in optimal proportions—equivalence. In the laboratory, titrating antigen against antibody reveals three phases:
- Antigen excess – early infection; free antigen detectable, no visible immune complexes.
- Equivalence zone – lattice formation; neither free antigen nor free antibody detected (e.g., hepatitis B “window period”).
- Antibody excess – later in infection; free antibody present, complexes form.
Clinical correlate: In HBV infection, HBsAg disappears during the window (equivalence) as HBsAb appears; this phase may be serologically “silent” for both markers.
🧪 Monoclonal vs polyclonal antibodies
- Polyclonal: derived from multiple B‑cell clones; recognize various epitopes on an antigen. Naturally produced during infection or by immunizing animals (rabbit, goat).
- Monoclonal: derived from a single clone; identical specificity for one epitope. Produced via hybridoma technology. Used in therapy (e.g., rituximab, trastuzumab) and diagnostics.
💊 Clinical pearl: Monoclonal antibodies are the backbone of many targeted cancer and autoimmune therapies; their specificity reduces off‑target effects.
📊 Direct vs indirect serologic tests
Direct – uses known antibody to detect unknown antigen (e.g., direct fluorescent antibody for Pneumocystis, viral antigens). Rapid, qualitative, used for screening.
Indirect – uses patient serum (antibody) to detect antigen; quantifies antibody titers. More specific; serial dilutions yield titer (reciprocal of highest dilution that gives a positive reaction).
Titers are used to follow disease progression (e.g., rising IgG titer) or confirm exposure.
🧫 Agglutination & Coombs tests
Agglutination
- Particulate antigens (RBCs, latex beads) cross‑linked by antibodies → visible clumping.
- Latex agglutination: used for CSF pathogens (meningococcus, H. influenzae, pneumococcus, Cryptococcus).
- RBC agglutination: blood typing, monospot (EBV), Coombs test.
Coombs tests
- Direct Coombs: detects antibodies already bound to RBCs in vivo (e.g., Rh incompatibility, autoimmune hemolytic anemia).
- Indirect Coombs: detects free antibodies in serum (e.g., Rh‑negative mother with anti‑Rh IgG, transfusion pre‑testing).
📌 USMLE: Direct Coombs + in warm autoimmune hemolytic anemia and hemolytic disease of the newborn (RhD). Indirect Coombs used for prenatal screening and cross‑matching.
🩸 ABO blood typing
- ABO antigens are glycoproteins on RBCs and endothelial cells.
- Natural isohemagglutinins (IgM) are produced against A/B antigens not present on self (due to cross‑reaction with intestinal flora).
- ABO incompatibility → hyperacute graft rejection; ABO typing is mandatory before transplantation.
| Blood type | Antigen on RBC | Serum antibodies (IgM) |
|---|---|---|
| A | A | anti‑B |
| B | B | anti‑A |
| AB | A and B | none |
| O | none | anti‑A and anti‑B |
🔦 Labeled antibody systems
- Direct fluorescent antibody (DFA): fluorescent‑labeled antibody directly detects antigen in patient tissue (e.g., rabies, HSV, RSV).
- Indirect fluorescent antibody (IFA): patient antibody binds to fixed antigen, then detected with fluorescent anti‑human Ig. Used for autoantibodies (ANA, anti‑dsDNA).
Comparison: DFA = “antigen detection”, IFA = “antibody detection” (patient serum).
🧪 ELISA & flow cytometry (FACS)
ELISA (enzyme‑linked immunosorbent assay)
- Extremely sensitive (detects picogram amounts). Uses enzyme‑labeled antibody and chromogenic substrate → color change.
- Common application: HIV screening (p24 antigen coated plate; patient serum + enzyme‑labeled anti‑human Ig).
- Can be direct or indirect; used for hormones, drugs, tumor markers, autoantibodies.
FACS (fluorescence‑activated cell sorting)
- Analyzes and sorts cells based on surface markers using fluorescently labeled antibodies.
- Generates scatter plots; each dot represents a cell with specific fluorescence intensity and color.
- Widely used in immunophenotyping (CD4/CD8 counts, leukemia/lymphoma diagnosis).
🧬 FACS high‑yield: CD4 count by flow cytometry is essential for staging HIV; abnormal populations (e.g., light chain restriction) suggest clonal B‑cell disorders.
⚡ High‑yield takeaways
- IgM = primary response, does not cross placenta → neonatal IgM = congenital infection.
- IgG = secondary response, crosses placenta, provides passive immunity.
- Zone of equivalence = maximal precipitation; HBV window period = equivalence (HBsAg and HBsAb both negative).
- Direct Coombs = detects antibody on RBCs (e.g., HDN, AIHA). Indirect Coombs = detects free antibody in serum (screening, crossmatch).
- ELISA = highly sensitive, enzyme‑mediated color change; HIV screening uses p24 antigen.
- FACS = cell sorting based on fluorescent markers; key for immunophenotyping.
- Monoclonal antibodies are used therapeutically; polyclonal sera are used in many diagnostic assays.
📖 Remember: serologic diagnosis is based on the principle of specific antigen‑antibody binding—understanding the kinetics and isotypes is essential for interpreting lab results.