Immunity to Viral Infections
- Immunity to viral infections involves the body’s defense mechanisms, which work together to recognize, neutralize, and eliminate viruses that invade the body.
- The immune response is complex and involves several layers of defense, including innate immunity (the body’s first line of defense) and adaptive immunity (the targeted immune response).
- Below is a more detailed breakdown of each aspect of immunity against viral infections.
Innate Immunity
Innate immunity is the body’s immediate, nonspecific response to viral infections. It is the first defense against pathogens and provides rapid but generalized protection. Unlike adaptive immunity, innate immunity does not “remember” previous infections but responds uniformly to different pathogens.
Components of Innate Immunity:
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Physical Barriers: The skin and mucosal membranes are the body’s first line of defense. The skin acts as a physical barrier, preventing viruses from entering the body. Mucosal membranes in the respiratory, gastrointestinal, and urogenital tracts contain secretions (e.g., mucus) that trap viral particles. Cilia in the respiratory tract also help move mucus and trapped viruses out of the body.
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Phagocytic Cells:
- Macrophages and dendritic cells are white blood cells that detect viruses using pattern recognition receptors (PRRs), which identify viral components like proteins or RNA. Once they identify a virus, these cells engulf and digest the virus (a process called phagocytosis).
- Neutrophils also play a role in this process and are the first responders to sites of infection.
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Natural Killer (NK) Cells:
- These cells can identify and kill virus-infected cells without the need for prior sensitization. NK cells target infected cells by recognizing changes in the surface of infected cells, such as the absence of “self” molecules (MHC class I molecules) on the cell surface.
- NK cells induce apoptosis (programmed cell death) in infected cells, preventing the virus from replicating further.
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Interferons:
- Interferons (IFNs) are cytokines released by infected cells. They play a crucial role in the antiviral immune response by signaling nearby cells to heighten their antiviral defenses. Interferons help induce the expression of antiviral proteins that inhibit viral replication within cells.
- Type I Interferons (e.g., IFN-α and IFN-β) are particularly important in viral infections like influenza and Hepatitis C.
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Complement System:
- The complement system consists of proteins that enhance the immune response. These proteins can bind to viral particles or infected cells and help destroy them. In some cases, the complement system marks the virus for destruction by phagocytic cells or directly causes the virus to rupture.
Function and Limitations:
Innate immunity provides an immediate response to infections but lacks specificity, meaning it does not tailor its response to each virus. Though effective at stopping viral spread in some cases, it can be overwhelmed by rapidly replicating viruses.
Adaptive Immunity
Adaptive immunity is the body’s more specific and targeted response to viral infections. It takes longer to develop than innate immunity but is highly effective and provides long-term protection.
Components of Adaptive Immunity:
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Humoral Immunity (Antibody-Mediated Immunity):
- B Cells: When a virus invades the body, B cells (a type of white blood cell) become activated. B cells recognize viral antigens (specific proteins or molecules from the virus) and differentiate into plasma cells that produce antibodies.
- Antibodies: These are proteins that specifically target the virus. They can neutralize viruses by binding to viral particles and preventing them from entering host cells. Antibodies also “tag” viruses for destruction by other immune cells.
- Neutralization: When antibodies bind to viral proteins on the virus surface, they prevent the virus from interacting with host cell receptors, thereby preventing infection.
- Opsonization: Antibodies can mark viruses for destruction by phagocytic cells, a process called opsonization.
- Memory B Cells: Once a virus has been cleared from the body, some B cells differentiate into memory B cells. These cells “remember” the virus and can quickly produce large quantities of antibodies if the body is exposed to the same virus again in the future.
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Cell-Mediated Immunity (T-Cell-Mediated Immunity):
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Cytotoxic T Cells (CD8+ T Cells): These cells directly kill infected cells. They do so by recognizing viral antigens displayed on the surface of infected cells in association with major histocompatibility complex (MHC) class I molecules. Upon recognizing these antigens, cytotoxic T cells kill the infected cells by inducing apoptosis.
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Helper T Cells (CD4+ T Cells): These cells do not kill infected cells but instead help activate and coordinate other immune responses. They assist B cells in producing antibodies and help cytotoxic T cells and macrophages function more effectively.
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Memory T Cells: Like memory B cells, memory T cells remain in the body after an infection. If the virus invades again, these memory cells can mount a faster and more effective immune response. Memory T cells are crucial in providing long-lasting immunity to viral infections.
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Function and Limitations:
Adaptive immunity is slower to respond but provides a highly specific, targeted defense and memory, allowing for quicker responses during re-infection. However, it takes several days to weeks to become fully activated, and some viruses, such as HIV, can evade the immune response, making immunity less effective.
Immunological Memory
Immunological memory is a hallmark of adaptive immunity and refers to the ability of the immune system to “remember” pathogens after the initial encounter. The immune system stores information about the virus in memory B cells and T cells, which can rapidly respond to the same virus if encountered again.
Aspects of Immunological Memory:
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Primary Immune Response: The first time the immune system encounters a virus, it takes time to generate an immune response. B cells begin producing antibodies, and cytotoxic T cells are activated. The response may take several days to weeks.
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Secondary Immune Response: If the virus infects the body again, memory cells generated during the primary infection respond much faster. The secondary immune response is quicker and more robust, often preventing the virus from causing significant disease or allowing for a rapid virus clearance.
- This concept is why people usually only get certain viral infections (like chickenpox) once and why vaccines are effective — they mimic the initial infection, generating memory cells that protect the body in case of future exposure.
Immunity through Vaccination
Vaccination exploits the body’s adaptive immune system to build immunity against viruses without causing disease. Vaccines contain inactivated or weakened versions of the virus (or parts of it) that stimulate the immune system to produce antibodies and memory cells.
Types of Vaccines:
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Inactivated or Killed Virus Vaccines: These vaccines contain viruses that have been killed or inactivated so they cannot cause illness. However, they can still stimulate the immune system to produce an immune response.
- Example: Polio vaccine.
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Live Attenuated Virus Vaccines: These vaccines contain a weakened form of the virus that cannot cause disease in healthy individuals but still triggers a strong immune response.
- Example: Measles, Mumps, Rubella (MMR) vaccine.
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Subunit Vaccines: These vaccines contain only a part of the virus, such as a protein, rather than the entire virus. These parts are usually sufficient to stimulate an immune response.
- Example: Hepatitis B vaccine, HPV vaccine.
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Messenger RNA (mRNA) Vaccines: These vaccines use genetic material (mRNA) to instruct cells to produce a specific viral protein. The immune system then learns to recognize and attack the virus if it encounters it in the future.
- Example: COVID-19 vaccines (e.g., Pfizer-BioNTech, Moderna).
Vaccines are crucial in preventing viral diseases by “teaching” the immune system how to recognize and respond to specific viruses without causing illness.
Immunological Evasion by Viruses
Many viruses have evolved strategies to evade or escape the immune system. These mechanisms help viruses persist in the host, sometimes leading to chronic infections.
Examples of Immune Evasion Strategies:
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Antigenic Variation: Some viruses, like the influenza virus, regularly change the surface proteins that the immune system recognizes. This is known as antigenic drift (a gradual change in viral antigens) or antigenic shift (a major change in viral antigens), allowing the virus to evade antibody recognition.
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Immune Suppression: Viruses such as HIV attack and destroy immune cells (particularly CD4+ T cells), weakening the immune system’s ability to respond to infections.
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Latency: Certain viruses, such as Herpes simplex virus (HSV) and Varicella-zoster virus (which causes chickenpox), can remain dormant (latent) in the body after the initial infection. These viruses can reactivate later, causing recurrent disease.
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Blocking Immune Responses: Some viruses can inhibit the host’s immune signalling pathways, preventing the immune system from recognizing or attacking the infected cells effectively. For example, cytomegalovirus (CMV) can interfere with the presentation of viral antigens to immune cells, preventing the activation of an immune response.
Factors Affecting Immunity to Viral Infections
Several factors influence how well the immune system can defend against viral infections:
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Age: Young children and the elderly tend to have weaker immune responses. Newborns have passive immunity from their mothers but require time to develop their own adaptive immunity. Older adults often have a weakened immune system, making them more susceptible to infections.
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Nutritional Status: Malnutrition or lacking essential nutrients (e.g., vitamins A, C, D, and zinc) can impair immune function and increase infection vulnerability.
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Immunocompromised States: People with weakened immune systems, such as those with HIV/AIDS, cancer, or those on immunosuppressive treatments, have a reduced ability to fight off viral infections.
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Previous Exposure: Individuals previously infected with or vaccinated against a virus may have pre-existing immunity, which allows for a quicker and more efficient immune response upon re-exposure.