Serological Diagnosis: Antibody and Antigen Detection

Serological diagnosis uses immunological reactions to detect either virus-specific antibodies produced by the host or viral antigens present in a specimen. By exploiting the specificity of antibody-antigen binding, serology can confirm exposure to a virus, stage an infection through antibody class and titre, or directly demonstrate viral proteins, often with rapid and scalable assay formats.

Definition

Serological diagnosis is the laboratory detection of virus-specific antibodies (the host immune response) or of viral antigens, using antibody-antigen binding reactions read by enzymatic, fluorescent, or other labelled-detection systems.

Scope

This topic covers the principles of antibody and antigen detection in virology, the main assay platforms (enzyme immunoassays, immunoblot, lateral-flow rapid tests, neutralisation), and the interpretation of antibody class and titre in relation to the timing of infection. It is a methodological reference and does not provide test-ordering protocols or clinical management advice.

Core questions

• Should the assay detect the host's antibodies or the virus's antigens, and what does each imply about infection status?
• How do IgM and IgG, and changes in titre, indicate the timing or stage of infection?
• How are sensitivity, specificity, and cross-reactivity balanced in assay design?
• When is a confirmatory or neutralisation assay needed to validate a screening result?

Key concepts

• Antibody-antigen specificity
• Enzyme-linked immunosorbent assay (ELISA)
• Immunoblot (Western blot) confirmation
• Lateral-flow rapid antigen test
• IgM versus IgG and seroconversion
• Antibody titre
• Neutralisation assay
• Window period and cross-reactivity

Mechanisms

Serological assays rely on the specific binding between an antibody and its antigen. In antibody detection, immobilised viral antigen captures patient antibodies, which are then revealed by a labelled secondary reagent, as in the enzyme-linked immunosorbent assay. In antigen detection, the immobilised reagent is an antibody that captures viral protein from the specimen, the format used in many rapid lateral-flow tests. The class of antibody detected helps stage infection: IgM typically appears earlier and IgG persists, and a rising titre between paired sera supports recent infection. Screening immunoassays are often paired with more specific confirmatory methods such as immunoblot or neutralisation, the latter measuring functionally relevant antibodies that block viral infectivity. Each design balances sensitivity against specificity, with cross-reactivity among related viruses a recurring challenge.

Clinical relevance

Serology supports diagnosis of past or present viral infection, immune-status assessment, and seroprevalence surveys; antigen tests provide rapid direct detection at the point of care. This entry describes how these assays work and how their results are framed, including the limitation that antibodies may be absent early in infection (the window period); it is descriptive and not a basis for individual diagnostic or treatment decisions.

Epidemiology

Antibody-based serosurveys are a standard tool for estimating the cumulative proportion of a population exposed to a virus, complementing molecular surveillance that captures active infection. Rapid antigen tests expand testing capacity during outbreaks by enabling decentralised, fast detection.

History

Virological serology developed from early agglutination, complement-fixation, and neutralisation tests, and was transformed by the introduction of the enzyme-linked immunosorbent assay by Engvall and Perlmann in 1971, which made antibody and antigen measurement quantitative and scalable. Protein immunoblotting, described by Towbin and colleagues in 1979, added a specific confirmatory format still used for several viral infections.

Key figures

• Eva Engvall
• Peter Perlmann
• Albert Coons

Seminal works

• engvall-perlmann-1971
• towbin-1979

Frequently asked questions

What is the difference between detecting antibodies and detecting antigens?

Antibody detection measures the host's immune response to a virus and can indicate past or ongoing infection, while antigen detection demonstrates viral proteins directly and reflects current presence of the virus. The two answer different questions about infection status.

Why can a serology test be negative early in infection?

Antibodies take time to develop after exposure, so during this window period an antibody test may be negative even though the virus is present. Direct methods such as antigen or nucleic-acid detection are used to detect early infection.