Antigenic Shift vs Antigenic Drift- Definition and 16 Differences

Antigenic shift and antigenic drift are two mechanisms by which viruses can change their surface proteins (antigens) and evade the immune system of their hosts. Antigens are molecules that trigger an immune response when recognized by specific antibodies. Antibodies are proteins produced by the immune system that bind to antigens and neutralize or destroy them. By changing their antigens, viruses can escape the recognition of existing antibodies and infect new cells or hosts.

The following figure illustrates the difference between antigenic shift and antigenic drift in influenza viruses. Antigenic shift occurs when two or more different strains of a virus infect the same cell and exchange genetic material, resulting in a new strain with a different combination of surface antigens. Antigenic drift occurs when small mutations accumulate over time in the viral genome, leading to changes in the surface antigens that may escape the immune recognition.

Antigenic shift and antigenic drift are two mechanisms by which viruses can change their surface proteins (antigens) and evade the immune system of their hosts. However, they differ in several aspects, such as:

• Definition: Antigenic shift is a sudden and major change in the antigenic composition of a virus, resulting from the reassortment of genetic material from two or more different strains. Antigenic drift is a gradual and minor change in the antigenic composition of a virus, resulting from mutations in the viral genome during replication.
• Frequency: Antigenic shift is a rare event that occurs only in some viruses that have segmented genomes, such as influenza A and B viruses. Antigenic drift is a common event that occurs in all viruses that have RNA genomes, such as influenza A, B and C viruses, HIV, rhinoviruses, etc.
• Impact: Antigenic shift can cause a dramatic change in the antigenicity of a virus, leading to the emergence of new strains that can infect a large population of susceptible hosts and cause pandemics. Antigenic drift can cause a subtle change in the antigenicity of a virus, leading to the emergence of new variants that can escape the existing immunity and cause epidemics.
• Examples: A well-known example of antigenic shift is the 1918 Spanish flu pandemic, which was caused by an avian H1N1 strain that acquired human-adapted genes from another strain. Another example is the 2009 swine flu pandemic, which was caused by a swine H1N1 strain that acquired genes from avian, human and swine strains. Examples of antigenic drift include the seasonal influenza outbreaks that occur every year due to the accumulation of mutations in the hemagglutinin and neuraminidase proteins of influenza viruses. Another example is the evolution of HIV, which undergoes rapid antigenic drift due to its high mutation rate and generates diverse variants that can evade the host immune response.

Antigenic shift and drift are two mechanisms by which viruses can change their surface antigens and evade the host immune system. Antigenic shift involves a major change in the antigenic properties of a virus due to reassortment of genetic segments from different strains or species of viruses. Antigenic drift involves minor changes in the antigenic properties of a virus due to accumulation of mutations over time.

Some examples of viruses that undergo antigenic shift and drift are:

• Influenza viruses: These are segmented RNA viruses that belong to the Orthomyxoviridae family. They are classified into types A, B, and C based on their nucleoprotein and matrix protein antigens. Influenza A viruses are further subtyped based on their hemagglutinin (HA) and neuraminidase (NA) antigens, which are the main targets of the host immune response. Influenza A viruses can infect humans, birds, pigs, and other animals, and can exchange genetic segments with other influenza A viruses through a process called reassortment. This can result in antigenic shift, which leads to the emergence of new influenza A subtypes that can cause pandemics. For example, the 1918 Spanish flu pandemic was caused by an H1N1 subtype that originated from an avian strain. The 2009 swine flu pandemic was caused by an H1N1 subtype that originated from a reassortment of human, avian, and swine strains. Influenza B and C viruses can only infect humans and do not undergo antigenic shift. However, all influenza viruses can undergo antigenic drift due to mutations in their HA and NA genes. This leads to the emergence of new variants that can escape the existing immunity and cause seasonal epidemics.
• HIV (human immunodeficiency virus): This is a retrovirus that belongs to the Lentivirus genus. It infects human CD4+ T cells and causes AIDS (acquired immunodeficiency syndrome). HIV has a high mutation rate due to the error-prone reverse transcriptase enzyme that copies its RNA genome into DNA. This results in antigenic drift, which leads to the generation of diverse viral variants within an infected individual and among different individuals. HIV also has a high recombination rate due to the presence of two copies of its genome in each virion. This can result in antigenic shift, which leads to the generation of new viral strains with different combinations of genes. Antigenic drift and shift contribute to the evasion of HIV from the host immune system and antiretroviral drugs.
• Rhinoviruses: These are non-segmented RNA viruses that belong to the Picornaviridae family. They are the most common cause of common colds in humans. Rhinoviruses have more than 100 serotypes based on their capsid protein antigens, which are recognized by neutralizing antibodies. Rhinoviruses can undergo antigenic drift due to mutations in their capsid genes. This leads to the emergence of new variants that can escape the existing immunity and cause recurrent infections. Rhinoviruses do not undergo antigenic shift because they do not exchange genetic segments with other viruses.
• Oncogenic viruses: These are viruses that can cause cancer in humans or animals by integrating their DNA or RNA into the host genome and altering the expression or function of cellular genes involved in cell growth, differentiation, or apoptosis. Some examples of oncogenic viruses are human papillomaviruses (HPVs), Epstein-Barr virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV), human T-cell leukemia virus type 1 (HTLV-1), Kaposi`s sarcoma-associated herpesvirus (KSHV), and Merkel cell polyomavirus (MCPyV). Oncogenic viruses can undergo antigenic drift due to mutations in their viral genes or host genes that they regulate. This can result in changes in their antigenicity or oncogenicity. Oncogenic viruses do not undergo antigenic shift because they do not exchange genetic segments with other viruses.

These are some examples of how antigenic shift and drift can affect the evolution and pathogenesis of various viruses. Understanding these mechanisms can help us develop better strategies for prevention, diagnosis, treatment, and control of viral diseases.

If you are preparing for the USMLE Step 1 exam, you may find the following resources helpful to review the concepts of antigenic shift and antigenic drift:

In this article, we have learned about the definitions and differences between antigenic shift and antigenic drift, two mechanisms of viral evolution that can lead to new strains of viruses. We have also seen some examples of how these processes have affected various viruses, such as influenza, HIV, rhinoviruses, and oncogenic viruses. Antigenic shift and drift are important topics to understand for USMLE Step 1 preparation, as they can help us explain the epidemiology, pathogenesis, and prevention of viral infections. We hope you have found this article informative and useful for your studies. If you want to learn more about antigenic shift and drift, you can check out the following videos and books:

Thank you for reading this article. Please leave your comments and feedback below. We would love to hear from you! 😊

Antigenic shift and antigenic drift are two mechanisms by which viruses can change their surface antigens and evade the host immune system. Antigenic shift involves the reassortment of genetic segments between different strains of the same or different virus species, resulting in a new combination of antigens. Antigenic drift involves the accumulation of point mutations in the viral genome, resulting in minor changes in the antigenic properties of the virus. Both processes can lead to the emergence of new viral variants that can cause epidemics or pandemics in humans and animals.

Antigenic shift and antigenic drift are important concepts to understand for medical students, especially those preparing for the USMLE Step 1 exam. They can help explain the epidemiology and pathogenesis of various viral infections, such as influenza, HIV, rhinovirus, and some oncogenic viruses. They can also inform the development and implementation of effective prevention and control strategies, such as vaccination and antiviral therapy.

To learn more about antigenic shift and antigenic drift, you can watch some USMLE Step 1 Preparation videos and read some books that cover this topic in detail. Here are some recommended resources:

We hope you enjoyed this article and learned something new about antigenic shift and antigenic drift. If you have any questions or feedback, please leave a comment below. Thank you for reading! 😊

We are Compiling this Section. Thanks for your understanding.