Patients with antibody to the following antigen are immune to hepatitis B

HBsAg variants

HBsAg variants arise as an escape mechanism during infection in the presence of anti-HBs. In babies given hyperimmune hepatitis B globulin (HBIG) and active immunization to reduce the risk of infection, but who are exposed to maternal virus during birth, the immune selection pressure from the HBIG and vaccine may result in the selection of HBsAg escape mutants. This is seen particularly in countries where universal childhood HBV vaccination has been practiced for many years, leading to breakthrough infection despite vaccination. Another scenario where immune selection pressure is intense is following liver transplantation for chronic hepatitis B. Immune selection pressure is exerted through the post-transplantation use of HBIG to protect the new liver from being re-infected with HBV.

HBsAg escape mutants mainly affect the ‘a’ determinant of HBsAg, the principal target of anti-HBs. The most common mutation observed is in position 145 of the HBsAg protein with an amino acid change from glycine to arginine (G145R). HBsAg escape mutants are transmissible and the widespread occurrence of HBsAg mutants would create considerable problems for the hepatitis B vaccination programme. In addition, many existing diagnostic assays for HBsAg use specific monoclonal antibodies that detect epitopes associated with the ‘a’ determinant. The presence of such mutations could lead to false negative diagnostic results when the monoclonal antibodies fail to bind to the mutated epitopes. Some cases of so-called ‘occult HBV infection’ are in fact due to false negative HBsAg, with detectable HBV DNA and no detectable antigenaemia.

Screening

Serum HBsAg and anti-HBs are the most useful screening tests for chronic HBV infection or immunity to HBV. HBsAg is present in most chronically infected persons, even when HBV DNA levels are undetectable. Lack of anti-HBs in an HBsAg-negative person indicates susceptibility to HBV infection.

Routine HBsAg screening is recommended for the following people: (1) all pregnant women; (2) all neonates born to HBsAg-positive women after completing the vaccination series; (3) immigrants from regions of high or intermediate HBV endemicity (e.g., most areas of Asia, Africa, the Middle East, and Pacific Islands); (4) U.S. born people, who were not vaccinated as infants, whose parents were born in regions with HBsAg prevalence ≥8%; (5) people with elevated serum ALT/AST levels of unknown etiology; (6) people with behavioral risks including MSM and injection drug users; and (7) and hemodialysis patients.69

Screening for HBsAg is recommended at the first prenatal visit for all pregnant women.109 Women in labor without HBsAg test information should have HBsAg serology upon arrival. In addition, pretested women who have a history of high-risk behaviors (e.g., HBsAg-positive sexual partner, injection-drug use) should be retested at the onset of labor. A copy of the original HBsAg test result should be placed in the infant's medical chart (in addition to the mother's chart) to avoid errors in communication or interpretation of the laboratory report.110

All HBsAg-positive tests should be reported to the state or local health department, including HBsAg-positive tests from pregnant women for referral to the Perinatal Hepatitis B Prevention Program.

Laboratory diagnosis

HBsAg is the most important serum marker for diagnosing HBV infection. The presence of HBsAg indicates infection and the person is infected as long as HBsAg or HBV DNA can be detected in the blood. In addition to the S antigen, the presence or absence of HBeAg in the serum can determine severe or mild infection. The presence of this antigen indicates a high infection in the individual, while the presence of the e antibody indicates a low or no infection in the blood. However mutations in the virus that lead to the lack of core expression (core mutants) or pre-core mutants, despite the inability to detect HBeAg in some individuals, large amounts of the genome is present in the blood, so the presence of DNA is a more accurate measure of infectivity. Anti-HBc IgM is important in detecting acute infection, although it has also been seen in cases of active chronic infection (Fig. 2). The presence of this antibody in the absence of HBsAg indicates recovery from an acute illness and is one of the criteria for differentiation of immunity caused by infection or vaccination (Coffin et al., 2019; Gerlich, 2013).

HBsAg

HBsAg, an HBV viral coat antigen, is produced in large quantities in infected-cell cytoplasm and continues to be produced in patients with chronic, active HBV infection (Fig. 13.1). This has been used as the primary screening test. HBsAg can be identified in an infected donor’s serum or plasma by enzyme immunoassays (EIAs) using animal antibodies (anti-HBs) as solid phase capture reagent and conjugated anti-HBs as probe. Chemiluminescent labels have replaced enzyme conjugates and chromogenic detection methods in automated testing strategies.

Sensitivities and specificities of current testing methods for HBsAg are high, but prevalence of detectable HBsAg in blood donors is low, and thus positive predictive value is also low.

Confirmation of HBsAg and interpretation of results:

Donor with repeatedly reactive (RR) testing result for HBsAg may be confirmed by concurrent reactive HBV NAT. These donors are permanently deferred. However, if NAT is nonreactive, neutralization test is performed.

With neutralization testing, HBsAg RR donor is confirmed as positive when anti-HBsAg is added to the serum, and HBsAg signal decreases by ≥50% from the control signal (antibody not added), such that the antigen is neutralized. Confirmed donors are permanently deferred.

If donor serum is not neutralized but the donor is RR and anti-HBc–reactive on current or any other donation, the donor is permanently deferred.

If the donor’s HBsAg reactivity is unconfirmed and anti-HBc is nonreactive, then the donor may be retested after 8 weeks and reinstated if HBsAg and anti-HBc are nonreactive. This retest may occur as per routine testing of a new donation.

Laboratory Findings and Diagnostic Tests

Serum HBsAg and anti-HBs are the most useful screening tests for chronic HBV infection or immunity to HBV. HBsAg is present in most chronically infected persons. Lack of anti-HBs in an unvaccinated HBsAg-negative person indicates susceptibility to HBV infection.3

Routine screening for HBV infection is recommended for the following people: (1) all pregnant women; (2) all infants born to HBsAg-positive women after completing the vaccination series; (3) immigrants from regions of high or intermediate HBV endemicity (e.g., most areas of Asia, Africa, the Middle East, and Pacific Islands); (4) US-born people, who were not vaccinated as infants, whose parents were born in regions with HBsAg prevalence ≥8%; (5) people with elevated serum ALT or aspartate transaminase levels of unknown etiology; (6) people with behavioral risks, including MSM and injection-drug users; (7) hemodialysis patients, (8) HIV-positive persons; (9) persons needing immunosuppressive therapy; (10) donors of blood, plasma, organs, tissues, or semen; (11) household, needle-sharing, or sexual contacts of persons known to be HBsAg positive; and (12) persons who are the source of blood or body fluids for exposures that might require postexposure prophylaxis.65

Screening for HBsAg is recommended at the first prenatal visit for all pregnant women.3,109 Women in labor without HBsAg test information should have HBsAg serology on arrival. In addition, pretested women who have a history of certain risk factors (i.e., HBsAg-positive sexual partner or more than one sexual partner in the previous 6 months, evaluation or treatment for a sexually transmitted disease, injection-drug use, and clinical hepatitis) should be retested at the time of admission to the hospital for delivery.3,110

Interpretation of Diagnostic Tests

Hepatitis B surface antigen (HBsAg) is the first marker of HBV detectable in serum in acute infection. By the time clinical and biochemical hepatitis is present after an incubation period of up to 140 days, other serologic markers of HBV infection appear—including antibody to HBV core antigen (anti-HBc). Hepatitis B core antigen, a marker of viral replication found in infected hepatocytes, does not circulate in serum. However, its corresponding antibody (anti-HBc) does. Documented HBsAg positivity in serum for 6 or more months suggests chronic HBV with a low likelihood of subsequent spontaneous resolution. Chronic HBV is diagnosed by the absence of IgM anti-HBc antibody. IgM anti-HBc antibody is a marker of acute or recent acute hepatitis B and is detectable for 6 months after infection, whereas IgG anti-HBc is lifelong. If acute HBV resolves, neutralizing antibody against HBsAg (anti-HBs) develops. If HBV infection becomes chronic, other HBV markers—including HBV viremia (HBV DNA) and hepatitis e antigen (HBeAg)—should be sought. Both of these markers imply viral replication and thus greater infectivity, although any patient who is HBsAg positive is potentially infectious.

Diagnosis of Acute and Chronic Hepatitis B

HBsAg is the first serologic marker to appear and may be detected within 1 to 2 weeks after exposure. It precedes the development of symptoms by an average of 4 weeks.106 The presence of HBsAg indicates ongoing infection. Qualitative but not quantitative methods are used by most clinical laboratories because the amount of antigen does not correlate with disease activity or with the presence of an acute or chronic infection.28 Some symptomatic patients may have self-limited, acute HBV infection without detectable HBsAg. These patients, up to 9% in some studies, have other detectable markers of infection.106 HBeAg appears virtually simultaneously, peaks, and then declines in parallel with HBsAg. It usually disappears before HBsAg does. Adult patients who remain persistently positive for HBeAg for more than 10 weeks are likely to become chronically infected. HBeAg indicates a high level of viral replication and infectivity. Most patients with nondetectable HBeAg have resolving, minimal, or no active liver disease.28 Precore mutants of HBV do not express HBeAg; they may be responsible for a more severe course and, in some cases, fulminant disease. Serum aminotransferase levels become elevated but are nonspecific. They begin to increase just before the development of symptoms and then peak (sometimes 20 or more times higher than normal) with the development of jaundice.

The diagnosis of chronic HBV infection is based on the persistence of appropriate markers for at least 6 months or on detection of these markers in a child who on initial presentation has historic or physical evidence of long-standing infection. This information is summarized in Fig. 75.5; also see Fig. 75.3 and Tables 75.3 and 75.5.

The third marker of infection is HBV-DNA, which appears with HBsAg, peaks with the onset of symptoms, and then declines. Anti-HBc is the last serologic marker to appear. It can usually be detected 3 to 5 weeks after the appearance of HBsAg but before the onset of symptoms, and it persists for life. The presence of anti-HBc indicates ongoing or past infection. Anti-HBc does not appear after HBV vaccination and, in the presence of anti-HBs, is therefore helpful in distinguishing immunity due to vaccination from that due to natural infection.

Anti-HBs indicates resolving or past infection or successful immunization with vaccine and confers protective immunity. In the majority of patients with self-limited infection, it can be detected only after HBsAg becomes undetectable. In a minority of patients with serum sickness–like symptoms, anti-HBs may appear before the onset of clinical symptoms.107,108 A “window” of variable duration has been described in some patients, during which HBsAg has disappeared and anti-HBs cannot yet be detected.106 The determination of anti-HBc may be helpful in these instances.

Antibody to HBeAg (anti-HBe) appears after HBeAg becomes undetectable and persists for 1 to 2 years after the resolution of hepatitis. The markers of HBV and their interpretation are summarized in Table 75.3.

Vaccines

HBsAg, derived from infected serum or produced by recombinant DNA technology, has been shown to induce protective immunity against HBV infection. These vaccines are now in widespread use throughout the world. When used to prevent neonatal transmission from infected mothers, the best protection is achieved when given with hyperimmune globulin.

Although the majority of patients vaccinated with HBsAg are protected from further infection, 2.5–5% of immunocompetent adults do not respond, probably for genetic reasons. This percentage is greater in some high-risk groups of patients, such as hemodialysis patients, in which the failure rate is approximately 40%: this probably represents a secondary immune deficiency due to the renal disease.

There has been considerable debate on whether the middle pre-S2-bearing and large pre-S1-bearing polypeptides should be added to existing vaccines. Experiments in mice suggested that both the pre-S1 and -S2 regions can recruit T cell help independently for the production of anti-HBsAg. It has also been suggested that pre-S1 is the region of the virus which binds to the hepatocyte during infection: antibodies to this region might be virus neutralizing and, therefore, inclusion of the pre-S1 region in a vaccine may be desirable.

The antibodies to HBsAg in convalescent serum (>80%) bind predominantly to the common ‘a’ determinant epitopes found in the hydrophilic region of the polypeptide between amino acids 124 and 147. The carboxy-terminal region of HBsAg has 14 cysteine residues and the antigenicity of this protein is highly dependent on its conformational structure. Using two cyclical peptides analogs of amino acids 124–137 and 139–147 and a panel of monoclonal antibodies binding to these regions, it has been possible to show in chimpanzees that antibody to the region aa124–137 of HBsAg prevents infection. Antibodies to this region, as well as to other epitopes on the surface antigen gene encoded polypeptide, are present in the serum of patients convalescent from HBV infection and in normal subjects immunized with plasma-derived and recombinant DNA-produced vaccine.

What is immunity to hepatitis B?

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Which antibody gives protection against hepatitis B virus?