Adaptive and Innate Immune Responses to Viruses & Vaccination: Study Notes

Immune Responses to Intracellular Viruses

Innate Immune Response

The innate immune system provides the first line of defense against intracellular viruses through rapid, non-specific mechanisms. Key components include interferons (IFNs), natural killer (NK) cells, and inflammation.

• Interferons (IFNs): These are cytokines produced by virus-infected cells. They induce an antiviral state in neighboring cells, upregulate antigen presentation, and activate immune cells.

• Natural Killer (NK) Cells: NK cells are cytotoxic lymphocytes that recognize and kill virus-infected cells, especially those with reduced MHC class I expression.

• Inflammation: Infection triggers the release of cytokines and chemokines, recruiting immune cells to the site of infection and promoting the elimination of pathogens.

Example: During influenza infection, infected epithelial cells release IFN-α and IFN-β, which activate NK cells and induce an antiviral state in neighboring cells.

Natural Killer (NK) Cells

Role in Viral Immunity

NK cells are a critical component of the innate immune response, particularly against viruses that evade detection by cytotoxic T cells.

• Activation: NK cells are activated by the absence or downregulation of MHC class I molecules on infected cells.

• Functions: They release cytotoxic granules containing perforin and granzymes, which induce apoptosis in virus-infected cells.

• Perforin: Forms pores in the target cell membrane.

• Granzymes: Enter the target cell through these pores and trigger programmed cell death (apoptosis).

Example: Herpesviruses often downregulate MHC class I to evade T cells, making them susceptible to NK cell-mediated killing.

Inflammation and Cytokines

Role in Immune Response

Inflammation is a hallmark of the innate immune response, characterized by redness, heat, swelling, and pain at the site of infection.

• Cytokines: Small proteins such as interleukins (ILs), tumor necrosis factor (TNF), and interferons (IFNs) that mediate and regulate immunity, inflammation, and hematopoiesis.

• Function: Cytokines recruit immune cells, enhance phagocytosis, and promote the clearance of pathogens.

Example: IL-1 and TNF-α are released during viral infection, leading to fever and recruitment of immune cells.

Activation of CD8+ T Cells (Cytotoxic T Lymphocytes)

Process and Function

CD8+ T cells, also known as cytotoxic T lymphocytes (CTLs), are essential for the adaptive immune response to viruses.

• Activation: Requires antigen presentation by dendritic cells via MHC class I molecules and co-stimulatory signals.

• Effector Function: Once activated, CD8+ T cells recognize and kill virus-infected cells by releasing perforin and granzymes.

• Detection: T cell receptors (TCRs) on CD8+ T cells specifically bind to viral peptides presented on MHC class I molecules.

Example: In HIV infection, CD8+ T cells target and destroy infected CD4+ T cells presenting viral antigens.

Activation of T-Dependent B Cells and Antibody-Mediated Protection

Mechanism and Importance

B cells can be activated with the help of T helper (Th) cells, leading to the production of high-affinity, class-switched antibodies.

• T-Dependent Activation: B cells present antigen to Th cells via MHC class II. Th cells provide co-stimulatory signals (e.g., CD40L-CD40 interaction) and cytokines.

• Antibody Production: Activated B cells differentiate into plasma cells that secrete antibodies specific to the pathogen.

• Protection: Antibodies neutralize pathogens, opsonize for phagocytosis, and activate complement.

Example: After influenza vaccination, B cells produce neutralizing antibodies that prevent viral entry into host cells.

Vaccines: Types, Administration, and Effectiveness

Current Vaccine Example

Vaccines are biological preparations that provide acquired immunity to specific infectious diseases. They can be live-attenuated, inactivated, subunit, or mRNA-based.

• Type: mRNA vaccine (e.g., Pfizer-BioNTech COVID-19 vaccine)

• Administration Schedule: Two doses, 3-4 weeks apart, with possible booster doses

• Immunity Generated: Induces both humoral (antibody) and cellular (T cell) immune responses

• Effectiveness: High efficacy in preventing symptomatic COVID-19 and severe disease

Example: The Pfizer-BioNTech COVID-19 vaccine uses lipid nanoparticles to deliver mRNA encoding the SARS-CoV-2 spike protein.

Vaccine Misinformation and the Importance of High Vaccination Rates

Common Myths and Scientific Explanations

Vaccine misinformation can undermine public health efforts. Addressing myths with scientific evidence is crucial for maintaining high vaccination rates and community protection.

Myth	Scientific Explanation
mRNA vaccines alter your DNA	mRNA does not enter the cell nucleus and cannot integrate into DNA. It is degraded after protein translation.
Vaccines cause autism	Extensive studies show no link between vaccines and autism. The original study suggesting this was retracted due to fraud.
Natural infection is better than vaccination	Vaccines provide immunity without the risks of severe disease or complications from natural infection.
Vaccines contain harmful toxins	Vaccine ingredients are present in safe amounts and are rigorously tested for safety.
High vaccination rates are unnecessary if most people are healthy	High vaccination rates are essential for herd immunity, protecting those who cannot be vaccinated (e.g., immunocompromised individuals).

Importance of High Vaccination Rates: High coverage prevents outbreaks, protects vulnerable populations, and can lead to the eradication of diseases (e.g., smallpox).

Additional info: Herd immunity occurs when a sufficient proportion of the population is immune, reducing the likelihood of disease spread.