Antigens in disease diagnosis

By Acharya Tankeshwar •  Updated: 05/13/22 •  5 min read

Detection of specific antigens present in the sample (blood, urine, CSF, or stool) using immunologic methods such as precipitin tests, particle agglutination tests, immunofluorescence, immunochromatographic or enzyme immunoassays are the mainstay for the rapid diagnosis of various infections. These test methods use polyclonal and monoclonal antibodies as tools to detect specific antigens of the pathogen of interest.

Antigens are foreign substances, usually high molecular weight proteins or glycoproteins, that elicit the production of antibodies. One organism may contain different antigens that the host will recognize as foreign. Find more about antigens here

Numerous immunologic methods are used for the rapid detection of bacteria, fungi, parasites, and viruses. For example,

Blood sample and lab request for anti-HIV testing

Stool Antigen Test

Stool antigen tests (SATs) are noninvasive diagnostic modules for Helicobacter pylori infection. This test was introduced after urea breath test (UBT). Currently used stool antigen test for the diagnosis of H. pylori infection is based on either enzyme immunoassay (EIA) or on immunochromatography (ICA).

Urine Antigen Test

Urinary antigen testing has grown in popularity for several significant respiratory infections, particularly Legionella pneumophila (legionellosis), Streptococcus pneumoniae (pneumococcal pneumonia), and Histoplasma capsulatum (histoplasmosis). Though these organisms are not present in urine for culture, antigens shed by them get concentrated in the kidney and excreted in the urine. Soluble antigens from Streptococcus agalactiae and Haemophilus influenzae may concentrate in urine.

Urine antigens are then detected via an immunoassay such as an enzyme-linked immunosorbent assay (ELISA) or an immunochromatographic or lateral flow assay (LFA).

Antigens present in Cerebrospinal fluids (CSF)

Soluble capsular polysaccharide antigen produced by the most common etiologic agent of meningitis including the group B streptococcal polysaccharide are detected from CSF using latex agglutination, coagglutination, or commercial agglutination systems.
Polysaccharide capsular antigen of Cryptococcus neoformans can also be detected from CSF samples using commercial reagents.

Antigen testing in other samples:

About the antigens:

Some commonly tested antigens during disease diagnosis are:

Name of the Antigen Disease
P24 HIV Infection
Histidine rich protein 2 (HRP2) Falciparum malaria
NS1 antigen Dengue
HBsAg and HBeAg Hepatitis B Infection
Parasite lactate dehydrogenase (pLDH). Malaria
Aldolase Malaria

P24 Antigen

p24 is a capsid structural protein that makes up a protein ‘shell’ on the surface of the HIV virus. p24 test is generally only positive from about two to three weeks after infection with HIV.

Histidine rich protein 2 (HRP2)

HRP-II is an abundant protein expressed only by Plasmodium falciparum and is the target for the most commonly used RDTs.

Parasite lactate dehydrogenase (pLDH)

It is produced by asexual and sexual stages (gametocytes) of malaria parasites. Malarial test kits that are currently available detect pLDH from all four species of Plasmodium. They can distinguish P. falciparum from the non-falciparum species, but cannot distinguish between P. malariae, P. ovale, and P. vivax.

Microbiologist standing at desk and holding test tube rack while preparing blood samples for centrifuge


HBeAg is a hepatitis B viral protein. It is an indicator of active viral replication; this means the person infected with Hepatitis B can likely transmit the virus on to another person (i.e. the person is infectious).

HBsAg (Australia antigen)

HBsAg is the surface antigen of the hepatitis B virus (HBV). It indicates the current hepatitis B infection.

NS1 Antigen

NS1 tests detect the non-structural protein NS1 of dengue virus in the serum using synthetically labeled antibodies. NS1 is detectable during the acute phase of dengue virus infections.

NS1 tests can be as sensitive as molecular tests during the first 0-7 days of symptoms. After day 7, NS1 tests are not recommended. A positive NS1 test result is indicative of a dengue infection but does not provide serotype information.


Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please email at

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