Immunology Lab Quiz
ELISA plates, flow cytometry assays, antibody-antigen matching, serology, and vaccination controls.
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Immunology Lab Quiz: ELISA, Flow Cytometry, Antibody-Antigen Tests, Serology, and Vaccination Controls
Immunology is the science of the immune system. The immunology laboratory translates that science into clinical and research tools: tests that detect whether a patient has been infected by or vaccinated against a pathogen, assays that measure the strength of an immune response, methods that characterise the types of immune cells in a blood sample, and platforms that support the development and quality control of vaccines and immunotherapeutics. Understanding these tools is essential not just for immunologists but for any laboratory scientist working in clinical diagnostics, infectious disease, or vaccine science.
This quiz is designed for immunology students, clinical serology laboratory staff, research scientists in vaccine or drug development, and healthcare professionals who interpret immunological test results. The questions cover ELISA principles and variants, flow cytometry and cell phenotyping, antibody-antigen reactions in diagnostics including agglutination and precipitation tests, serology interpretation, and immunological controls in vaccination research.
Core Topics
ELISA: Principles and Variants
The enzyme-linked immunosorbent assay (ELISA) is one of the most widely used diagnostic and research tools in immunology. It detects and quantifies antigens or antibodies in a sample by exploiting the highly specific binding between an antibody and its target antigen, and linking that binding to an enzyme-catalysed colour reaction that can be measured by a spectrophotometer.
In a direct ELISA, the antigen is coated onto a microplate well, and a primary antibody conjugated directly to an enzyme is added. In an indirect ELISA, a secondary antibody conjugated to an enzyme detects the primary antibody bound to the antigen. This amplifies the signal and is used for detecting patient antibodies against a specific antigen (for example, antibodies against HIV, hepatitis B surface antigen, or SARS-CoV-2 spike protein). The sandwich ELISA captures the antigen between two antibodies: a capture antibody coated on the plate and a detection antibody in solution. This format is used to detect and quantify soluble antigens including cytokines and pathogen antigens in clinical samples. The competitive ELISA uses competition between the sample antigen and a labelled antigen for binding to a fixed quantity of antibody, and is used when the analyte is small and cannot accommodate two antibodies binding simultaneously.
In all ELISA formats, an enzyme substrate is added in the final step. The most commonly used enzyme is horseradish peroxidase (HRP), which converts a substrate (such as TMB, tetramethylbenzidine) from colourless to blue (and then yellow after stopping). The intensity of colour, measured as absorbance at 450 nm (OD450), is proportional to the amount of analyte present.
Flow Cytometry
Flow cytometry is a technology that measures multiple physical and chemical characteristics of individual cells as they flow in a stream through a laser beam. Cells are labelled with fluorescent antibodies specific to surface or intracellular markers. As each cell passes through the laser, it scatters light and emits fluorescence, which is detected by multiple photomultiplier tubes. The resulting data includes forward scatter (FSC, correlating with cell size), side scatter (SSC, correlating with cell granularity), and fluorescence intensity in multiple channels corresponding to different fluorochromes.
In clinical immunology, flow cytometry is used to count and classify blood cell populations, most importantly CD4+ T-cells (helper T-cells) and CD8+ T-cells (cytotoxic T-cells) in HIV-infected patients. The CD4 count is used to stage HIV disease and to guide decisions about antiretroviral therapy and prophylaxis against opportunistic infections. Flow cytometry is also used in haematology to diagnose leukaemia and lymphoma by identifying abnormal cell populations with unusual combinations of surface markers, and in research to characterise immune responses to vaccines or infections.
Antibody-Antigen Reactions in Diagnostics
Antibodies bind to their specific antigens with high affinity and specificity, and this interaction is the basis of many diagnostic tests. Agglutination tests detect antibodies that cause visible clumping of antigen-coated particles. In the Widal test (now largely replaced by better methods), serum from a patient suspected of having typhoid fever is mixed with Salmonella antigens: agglutination suggests the presence of antibodies. Latex agglutination tests coat latex particles with antigens or antibodies and are used for rapid detection of bacterial antigens (for example, Group B Streptococcus antigen in neonatal CSF) or antibodies in clinical samples.
Precipitation reactions occur when antibodies and soluble antigens combine to form an insoluble complex. The Ouchterlony double immunodiffusion test is a classic precipitation method used in research and reference serology. Immunofixation electrophoresis (IFE) is a precipitation technique used to detect and type monoclonal immunoglobulins (paraproteins) in the serum or urine of patients with multiple myeloma.
Serology: Antibody Testing in Infectious Disease
Serological tests detect antibodies against specific pathogens in patient serum or plasma. They are used for three main purposes: diagnosis of acute infection (detecting IgM antibodies, which appear early in infection), evidence of past infection or vaccine-induced immunity (detecting IgG antibodies, which persist long-term), and monitoring of chronic infections or treatment response (measuring antibody titres over time). The antibody titer is the highest dilution of serum at which a positive reaction is still detected, and a four-fold rise in titer between acute and convalescent samples is considered evidence of recent active infection.
IgM antibodies are the first antibodies produced in a primary immune response. Their presence suggests recent or current infection. IgG antibodies appear later but persist for years or lifelong after infection or vaccination. Distinguishing IgM from IgG is clinically important: IgM against toxoplasma during pregnancy suggests acute infection with risk to the fetus, while IgG alone indicates past immunity.
Frequently Asked Questions
What is ELISA and how does it work?
ELISA (enzyme-linked immunosorbent assay) is a plate-based assay technique used to detect and quantify specific antigens or antibodies in a sample. It works by exploiting the specific binding of an antibody to its antigen, then using an enzyme-linked detector system that produces a measurable colour change. The intensity of the colour is measured by a spectrophotometer and is proportional to the concentration of the target molecule. ELISA is used in clinical diagnostics for serological testing, viral antigen detection, cytokine quantification, and many other applications.
What is the difference between direct and indirect ELISA?
In a direct ELISA, the primary antibody is directly conjugated to an enzyme and binds to the antigen coated on the plate. In an indirect ELISA, an unconjugated primary antibody binds to the antigen, and then an enzyme-conjugated secondary antibody binds to the primary antibody. Indirect ELISA produces signal amplification (multiple secondary antibodies can bind to each primary antibody) and allows the same secondary antibody to be used across many different primary antibodies, reducing cost. Indirect ELISA is the format used for detecting patient antibodies against specific antigens.
What is flow cytometry used for?
Flow cytometry is used to rapidly measure multiple characteristics of individual cells in a liquid sample. Clinical applications include counting and classifying blood cell populations, CD4 T-cell counting for HIV monitoring, immunophenotyping for leukaemia and lymphoma diagnosis, detection of paroxysmal nocturnal haemoglobinuria (PNH), and monitoring of minimal residual disease (MRD) in haematological malignancies. Research applications include characterising immune cell populations, measuring cell proliferation and apoptosis, and analysing immune responses to vaccines or infections.
What are CD markers?
CD markers (cluster of differentiation markers) are cell surface proteins used to identify and classify immune cell populations. Over 400 CD markers have been identified in humans. CD4 is expressed on helper T-cells and is the receptor that HIV uses to enter cells. CD8 is expressed on cytotoxic T-cells. CD19 and CD20 are expressed on B-cells. CD56 marks natural killer cells. The expression profile of CD markers on a cell is called its immunophenotype and is determined by flow cytometry using fluorescently labelled antibodies.
What is agglutination in serology?
Agglutination is the visible clumping of particles (cells or antigen-coated beads) caused by antibody cross-linking. When antibodies bind to antigens on multiple particles simultaneously, they bridge the particles together into a visible aggregate. Agglutination tests are used for blood group typing (mixing red blood cells with anti-A and anti-B antibodies), rapid streptococcal antigen detection, Cryptococcus antigen detection, and many other applications. The degree of agglutination can be graded to provide a semi-quantitative result.
What is the complement fixation test?
The complement fixation test (CFT) is a serological method that detects antibodies against specific pathogens. It relies on the ability of antibody-antigen complexes to activate (fix) complement. The test uses a two-step system: in the first step, patient serum is mixed with a known antigen and complement; if antibodies to the antigen are present, they form immune complexes that consume the complement. In the second step, sensitised red blood cells are added: if complement has been consumed (positive result), the red blood cells do not lyse. If complement remains (negative result), the red blood cells lyse. The CFT has largely been replaced by ELISA in modern serology but is still used in some reference laboratories.
What is an antibody titer?
An antibody titer is the highest dilution of a patient’s serum at which a specific serological reaction is still detected. It is used to quantify the strength of an antibody response. A high titer indicates a strong immune response. A four-fold rise in titer between samples taken during acute illness and convalescence (2 to 4 weeks later) is considered diagnostic evidence of recent active infection with the tested pathogen. Titer measurements are used in monitoring immune responses to vaccines and in following the progression of some chronic infections.
What is the difference between IgM and IgG in serology?
IgM is the first antibody class produced in a primary immune response. It appears within days of infection, rises rapidly, and then falls over weeks. Its presence generally indicates recent or current infection. IgG appears later in the primary response but persists for years or lifelong, providing long-term immunological memory. In secondary responses (re-exposure or re-infection), IgG rises much faster and to higher levels than in the primary response. Distinguishing IgM from IgG is clinically important: IgM detection is used to diagnose acute infection, while IgG detection indicates past exposure or vaccine-induced immunity.
What does a western blot detect?
A western blot (immunoblot) detects specific proteins by first separating proteins from a sample by size using gel electrophoresis, transferring them to a membrane, and then probing with labelled antibodies. It is used as a confirmatory test in HIV diagnosis (after a positive ELISA screening test), for diagnosing Lyme disease (after a positive ELISA), and in research for detecting specific proteins in cell or tissue lysates. The western blot provides both confirmation of antibody specificity and information on the molecular weight of the target protein.
What is FACS?
FACS stands for Fluorescence-Activated Cell Sorting, which is a specialised type of flow cytometry that not only analyses cells but also physically sorts them into separate collection tubes based on their measured properties. A FACS instrument creates an electrostatic charge on individual droplets containing single cells and deflects them into the appropriate collection vessel using an electric field. FACS allows researchers to collect pure populations of specific cell types for further analysis, gene expression studies, or functional assays. The term FACS is often used colloquially to refer to flow cytometry in general, even when cell sorting is not being performed.