Infectious mononucleosis: antibodies to Epstein-Barr virus in blood

Infectious mononucleosis is a common systemic lymphoproliferative disorder most often caused by the Epstein-Barr virus. Toxoplasma gondii and other viruses (CMV, human immunodeficiency virus, and human herpes virus type 6, recognized as the cause of sudden exanthema) can cause clinically similar diseases. These same etiologic agents may presumably cause the development of chronic fatigue syndrome.

Epstein-Barr virus is a herpes virus that has a tropism for B-lymphocytes and persists in host cells for a long time as a latent infection. It is widespread throughout the world. In structure and size, the Epstein-Barr virus is indistinguishable from other herpes viruses, but differs significantly from them in antigenic properties. The virus has a membrane antigen (MA - membrane antigen), a nuclear antigen (EBNA - Epstein-Barris nucleic antigen) and an antigen of the viral capsid (VCA - virus capsid antigen).

Infection occurs when the virus is transmitted with saliva. When the Epstein-Barr virus enters the body, it infects the pharyngeal epithelium, causing inflammation and fever - typical clinical signs of the onset of infectious mononucleosis. The virus is strictly lymphotropic, attaching to the C3α receptor of the B-lymphocyte cell membrane, it causes proliferation of polyclonal B-lymphocytes with a corresponding increase in tonsils, systemic lymphadenopathy and splenomegaly. B-lymphocytes are transformed (acquire the ability to endlessly divide), and in the absence of an adequate cellular immune response, this process can evolve into a clearly malignant one (for example, in X-linked lymphoproliferative syndrome). If cellular immunity factors control the replication of the Epstein-Barr virus in the body, the clinical symptoms of infectious mononucleosis gradually disappear.

Like other herpesviruses, EBV can persist as a latent infection (its DNA is contained in the nucleus of a small number of B lymphocytes). Occasional asymptomatic reactivation of infection is common, with about 20% of healthy young adults excreting EBV in their saliva. Individuals with impaired cellular immunity (e.g., AIDS, ataxia-telangiectasia, transplant recipients) may develop overt reactive infection with hairy leukoplakia, interstitial pneumonitis, or monoclonal B-cell lymphoma. EBV has been implicated in the etiology of nasopharyngeal carcinoma and Burkitt's lymphoma.

One of the manifestations of infectious mononucleosis is the appearance of atypical lymphocytes in the peripheral blood (up to 10% of the total number of lymphocytes). Atypical lymphocytes are detected in the blood from the beginning of the period of clinical manifestations of the infection. Their content in the blood reaches a peak by the end of the 2nd or the beginning of the 3rd week and can remain at this level for up to 1.5-2 months, complete disappearance usually occurs by the beginning of the 4th month from the onset of the disease. The presence of atypical lymphocytes is a relatively insensitive sign of infection caused by the Epstein-Barr virus, but has a general specificity of about 95%.

The proliferation of polyclonal B lymphocytes in the infection caused by the Epstein-Barr virus generates a large number of various autoantibodies in the patient's body, such as IgM anti-i (cold agglutinin), rheumatoid factor, antinuclear antibodies. Most of the unusual Ig that appears in infectious mononucleosis are called Paul-Bunnell heterophile antibodies. These antibodies belong to the IgM class, they have an affinity for sheep and horse erythrocytes, and are not directed to any Epstein-Barr virus antigens. Heterophile antibodies are a random product of B-lymphoid proliferation (caused by the Epstein-Barr virus), they appear in the first week of infectious mononucleosis and gradually disappear during recovery, they are usually not detected after 3-6 months.

As the initial acute stage of infection becomes latent, Epstein-Barr virus genomes (unique antigens) appear in large quantities in all cells, and the nuclear antigen is released into the environment. In response to the antigen, specific antibodies are synthesized - valuable markers of the stage of the disease. Soon after infection, B-lymphocytes detect an early antigen (EA), a protein necessary for the replication of the Epstein-Barr virus (and not a structural viral component). Antibodies of the IgM and IgG classes are synthesized in the patient's body to the early antigen. Together with the complete Epstein-Barr virus virion, viral capsid antigens (VCA) and membrane antigen (MA) appear. As the infectious process subsides, a small percentage of Epstein-Barr virus-infected B-lymphocytes avoid immune destruction and retain the viral genome in a latent form. The Epstein-Barr virus nuclear antigen (EBNA) is responsible for its duplication and survival.

Laboratory tests can detect antibodies to various antigens.

Of the serological methods for diagnosing infectious mononucleosis, the most common is the Paul-Bunnell reaction (agglutination), aimed at identifying heterophilic antibodies in the serum. A titer of heterophilic antibodies of 1:224 or higher in the patient's blood serum is recognized as diagnostically significant, confirming the diagnosis of infectious mononucleosis. Heterophilic agglutination is positive in 60% of young people after 2 weeks and in 90% after 4 weeks from the onset of clinical manifestations of the disease. Therefore, to diagnose infectious mononucleosis, it is necessary to conduct several studies: in the first week of the disease (the reaction may be negative) and after 1-2 weeks (the reaction may become positive). The content of heterophilic antibodies decreases at the end of the acute period of the infectious process, but their titer can be determined within 9 months after the onset of clinical symptoms. The Paul-Bunnell reaction can turn from positive to negative, even against the background of residual hematological and clinical symptoms in the patient. The sensitivity of the method in adults is 98%, specificity - 99%. In children with infectious mononucleosis under 2 years of age, heterophile antibodies are detected only in 30% of patients, at the age of 2-4 years - in 75%, over 4 years - more than 90%. The sensitivity of the method in children is less than 70%, specificity - 20%. A decrease and then a repeated increase in the titer of heterophile antibodies can occur in response to another infection (most often with viral infections of the upper respiratory tract). The Paul-Bunnell reaction is non-specific for the Epstein-Barr virus. The titer of heterophile antibodies does not cross-react and does not correlate with specific antibodies to the Epstein-Barr virus, there is also no correlation with the severity of the disease. The test is useless for diagnosing chronic infectious mononucleosis (it is positive on average in only 10% of patients).

Titres of 1:56 and less can be found in healthy people and in patients with other diseases (rheumatoid arthritis, rubella). False positive test results are very rare.

Currently, the "single spot" method (slide agglutination) is used to determine antibodies to sheep red blood cells; it is used initially as a screening test. In terms of sensitivity, it is comparable to the Paul-Bunnell reaction. Slide tests can be false positive in approximately 2% of studies (in leukemia, malignant lymphoma, malaria, rubella, viral hepatitis, pancreatic carcinoma), and false negative in adults - in 5-7% of cases.

It should be noted that the range of diagnostic test systems produced by companies based on the determination of antibody titers is very wide, therefore it is necessary to focus on the diagnostic antibody titer indicated in the instructions for the test systems.

If heterophilic antibodies are not detected and the clinical picture of the disease corresponds to infectious mononucleosis, it is necessary to examine the blood serum for specific antibodies of the IgM and IgG classes. To detect specific antibodies to the Epstein-Barr virus, indirect immunofluorescence methods are used (allow detection of antibodies to EA and VCA antigens), anticomplement immunofluorescence (detects antibodies to EA, VCA and EBNA antigens) and ELISA.

Antibodies to the EA antigen D component (anti-EA-D) appear even during the latent period of primary infection and quickly disappear with recovery.

Antibodies to the EA antigen R component (anti-EA-R) can be detected 3-4 weeks after clinical manifestations of the disease. They persist in the blood serum for about a year, and are often detected in atypical or protracted infectious mononucleosis. These antibodies are usually found in Burkitt's lymphoma.

Antibodies to VCA class IgM (anti-VCA IgM) appear very early, usually before clinical symptoms, they are detected at the onset of the disease in 100% of cases. High titers occur at 1-6 weeks from the onset of infection, they begin to decrease from the 3rd week and usually disappear after 1-6 months. Anti-VCA IgM are almost always present in the serum during active infection, so the method of their detection is very sensitive and specific for an acute episode of infectious mononucleosis.

Antibodies to VCA class IgG (anti-VCA IgG) may appear early (at 1-4 weeks), their number reaches a peak by the 2nd month of the disease. At the onset of the disease, they are detected in 100% of cases. Only 20% of patients show a 4-fold increase in titer when examining paired sera. The titer decreases during recovery, but is detectable for several years after the infection, so it is useless for diagnosing infectious mononucleosis. The presence of anti-VCA IgG indicates the state after the infection and immunity.

Antibodies to EBNA (anti-EBNA) appear last of all, are rarely present in the acute phase of the disease. Their content increases during the recovery period (within 3-12 months), they can remain in the blood for many years after the disease. The absence of anti-EBNA in the presence of anti-VCA IgM and anti-EA IgM indicates a current infection. Detection of anti-EBNA after a previously negative reaction indicates an existing infection. Using the ELISA method, it is possible to simultaneously determine the presence of anti-EBNA classes IgM and IgG. If the amount of anti-EBNA IgM is greater than anti-EBNA IgG, an acute infection should be discussed, with the opposite ratio - a previously suffered one.

The presence of one or more of the following signs indicates acute primary infection:

• anti-VCA IgG (detected early, and later the content decreases);
• high titer (more than 1:320) or 4-fold increase in anti-VCA IgG titer during the course of the disease;
• transient increase in anti-EA-D titer (1:10 or more);
• early anti-VCA IgG without anti-EBNA, and later the emergence of anti-EBNA.

Acute or primary infection caused by the Epstein-Barr virus is excluded if the titers of anti-VCA IgG and anti-EBNA in the blood serum do not change when studied dynamically (during the acute period and during recovery).

The persistent presence of early antigen and anti-VCA IgG in high titers indicates a chronic phase of infection.

Detection of antibodies to the Epstein-Barr virus is used to diagnose infectious mononucleosis and chronic infections caused by the Epstein-Barr virus.

Antibodies to the Epstein-Barr virus can be detected in the following diseases: secondary immunodeficiency states, including HIV infection, nasopharyngeal carcinoma, Burkitt's lymphoma, CMV infection, syphilis, Lyme disease, brucellosis, etc.

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