When individuals are infected with influenza virus, antibodies are made against the various viral proteins. However, it is the antibodies made against the H protein that are neutralizing and the best index of protection. The antigenic properties of the influenza virus proteins are also important because they serve as the basis for the classification of influenza viruses.

 1. Types and subtypes: Influenza viruses are classified as types A, B, and C, depending on their inner proteins, mainly the M and NP proteins. Thus, all type A viruses share common internal antigens that are distinct from those shared by all type B viruses. Only the type A viruses are broken down into subtypes. The classification into subtypes depends on antigens associated with the outer viral proteins, H and N. Taking into consideration animal as well as human influenza viruses, 16 H and 9 N subtypes have been described. Subtypes of influenza viruses are, therefore, designated by the unique combinations of H and N antigens, for example: H1N1, H2N2, and H3N2.

 2. Antigenic variability of influenza viruses: In contrast to viruses such as polio or measles virus that have maintained antigenic stability since they were first isolated, influenza viruses have shown marked variation over the years in antigenic properties, specifically H and N proteins. Two distinct phenomena account for this observation: antigenic drift and antigenic shift.

a. Antigenic drift: This refers to minor antigenic changes in H and N proteins that occur each year. Antigenic drift does not involve a change in the viral subtype. This phenomenon can be easily explained by random mutations in viral RNA and single or a small number of amino acid substitutions in H and N proteins.

b. Antigenic shift: This phenomenon involves a much more dramatic change in the antigenic properties of the H and/or N proteins (Figure 1), and a change in subtype, for example, from H1N1 to H3N2. Antigenic shift occurs only infrequently, perhaps every 10 or 20 years. For example, the appearance of a new, extremely virulent H1N1 virus, due presumably to anti genic shift, probably accounted for the pandemic of 1918–1919 that resulted in the death of an estimated 20 million people worldwide, including more than 500,000 in the United States (Figure 2). In 1957, antigenic shift again occurred, and H1N1 virus was replaced by subtype H2N2. In 1968, H2N2 was replaced by H3N2. Since 1977, multiple sub types of influenza A have been circulating around the world. In most years, both type A and type B influenza viruses can be isolated from patients. Both type A and B viruses undergo anti genic drift, but only type A viruses show antigenic shift. H5N1 (an avian flu subtype) was first isolated in 1997 from a human. The virus affects individuals who live closely with domestic birds such as chickens. Passage from human to human has not been seen, but rather from bird to bird, including wild birds that migrate. The primary infections have been in Asia and the Middle East. Over 50 percent of reported cases have been fatal.

Fig1. Mechanism of antigenic shift in influenza virus.

Fig2. Time line showing the occurrence of some of the major outbreaks and antigenic shifts associated with type A influenza during the 1900s.

 c. Consequences of antigenic variation: When antigenic shift occurs, giving rise to a subtype of virus appears that has not been in circulation for many years, the immune systems of a large proportion of the population have never encountered that virus. Therefore, these individuals are immunologically unprotected, and the conditions are set for an influenza epidemic (disease prevalent in a community) or even pandemic (disease prevalent over a whole country or the world) as shown in Figure 2. Antigenic shift also means that the vaccine that was in use before the antigenic shift will not be effective in protecting against the new subtype of virus. Therefore, developing a new vaccine as quickly as possible that incorporates the new virus subtype becomes necessary.

 d. Molecular basis of antigenic variation: The dramatic changes associated with antigenic shift result from reassortment of viral RNA segments, a process observed with all RNA viruses having a segmented genome. Reassortment results when a cell is infected with two genetically distinct influenza viruses: The genomic RNAs of both parental viruses are replicated, and progeny viruses are assembled that contain genomic RNA segments from one of the parental viruses and other genomic segments from the second parent (see Figure 1). In this way, new viruses can be generated that differ from both par ents. Although all eight of the influenza virus RNA genome segments undergo reassortment, for antigenic shift to occur, it is the reassortment of the RNA segments that specify the mRNAs for H and N proteins (the proteins that define the antigenic subtypes) that is most critical. How does this reassortment occur? We know that influenza type A viruses are found in many different animals, including horses, pigs, and wild migrating waterfowl. Furthermore, it has been demonstrated that reassortment can occur between influenza A viruses that infect different animal and avian species. For example, pigs can be infected by human- and avian-specific influenza viruses. In environments where pigs, birds and humans coexist, it is possible for a pig to be simultaneously infected with multiple influenza subtypes. "Reassortants" can, therefore, be produced within one host animal (the pig), in which the mRNAs encoding the H and N antigens have been reassorted into unique combinations. The reassortant virus then has the potential to spread among humans, birds, and pigs.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تشارك بأكثر من 750 إصدارًا في معرض بغداد الدولي للكتاب

تيمنًا بذكرى ولادة الصادقَين (صلوات الله عليهما) شعبة التوجيه النسوي تنظم مسابقة ميدانية لزائرات مرقد أبي الفضل العباس (عليه السلام)

المجمع العلمي ينظّم أنشطةً قرآنية في محافظتي كربلاء والديوانية

العتبة العباسية المقدسة تختتم فعاليات مهرجان رحمة للعالمين الدولي الأوَّل

المزيد

الآخبار الصحية

تحذير من الإفراط في شربها.. متى تصبح الماتشا خطيرة؟

من بناء العضلات إلى "تسميم الجسم".. الوجه الآخر للبروتين ؟

دراسة جديدة: مخ الانسان يغتسل أثناء النوم

هل من الآمن شرب القهوة على معدة فارغة؟

المزيد

الآخبار التكنلوجية

مزايا مرآة الرؤية الخلفية ذاتية التعتيم في السيارات

دراسة.. كوكب غريب أمد الأرض بمقومات الحياة !

يهدد عرش هوليود.. أول فيلم رسوم متحركة بالذكاء الاصطناعي

أكبر تغيير منذ سنوات.. هل تكشف أبل النقاب عن "iPhone Air"؟

المزيد

مواضيع ذات صلة

Type II Antibody-Dependent, Complement-Mediated Cytotoxic Reactions

Immunologic Manifestations of Rubella infection

Antigen presentation

Cytokines

Structure of MHC Molecule

MHC Genes and Their Products

Macrophage

B Lymphocytes

T Lymphocytes

Immunologic Manifestations of Infectious Mononucleosis

Peripheral Lymphoid Organs

Central Lymphoid Organs