5. Which one of the following strategies do viruses employ to evade the human immune
system?
(a) Virus binds to TCR and blocks activation of T-cells,
(b) Virus infected cells show increased expression of pro-inflammatory cytokines,
(c) Virus infected cells show reduced expression of surface MHC Class I molecules,
(d) Viruses bind to surface Ig on B-cells and neutralize them,

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Article:

Introduction

Viruses have evolved a wide variety of strategies to evade the human immune system, allowing them to persist in the body and cause infection. The human immune system is adept at recognizing and attacking foreign invaders, including viruses. However, viruses have developed numerous mechanisms to evade detection and destruction by immune cells. One such strategy is by manipulating the immune system’s ability to recognize infected cells, allowing the virus to thrive undetected.

In this article, we will explore how viruses evade the human immune system, with a focus on the role of MHC Class I molecules, T-cells, and other immune evasion tactics.

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Immune Evasion Mechanisms Employed by Viruses

There are several strategies that viruses use to evade the immune system. These strategies are essential for their survival and replication within the host. One of the most effective mechanisms is altering how the immune system identifies infected cells.

1. Reducing Expression of MHC Class I Molecules

One of the key strategies used by many viruses is the reduction of MHC Class I molecule expression on the surface of infected cells. MHC Class I molecules are responsible for presenting viral peptides (fragments of proteins from viruses) on the surface of infected cells. This presentation allows cytotoxic T-cells (CD8+ T-cells) to recognize and kill the infected cells.

By reducing the expression of MHC Class I molecules, viruses can prevent T-cells from recognizing and targeting infected cells. This allows the virus to hide from the immune system, effectively evading immune surveillance. This mechanism is commonly seen in viruses like herpesviruses and HIV.

2. Virus Binding to TCR to Block T-Cell Activation

In addition to reducing MHC Class I expression, some viruses directly interfere with the function of T-cells. T-cells are essential for recognizing and responding to infected cells. Certain viruses can bind to the T-cell receptor (TCR), preventing the activation of T-cells. This can block the T-cell’s ability to respond to viral infections, further hindering the immune system’s ability to mount an effective defense.

While this strategy is not as common as MHC Class I downregulation, some viruses, such as HIV, are known to manipulate T-cell responses to suppress immune activation.

3. Increased Expression of Pro-Inflammatory Cytokines

While most viruses strive to avoid triggering immune responses, some may manipulate the immune system by increasing the production of pro-inflammatory cytokines. These cytokines are signaling molecules that can activate and recruit immune cells to the site of infection. However, in some cases, this excess inflammation may impair immune function or lead to tissue damage.

Viruses like herpes simplex virus (HSV) and influenza can trigger the production of pro-inflammatory cytokines to create a cytokine storm, which can overwhelm the immune system and lead to severe disease. This strategy can confuse the immune system and allow the virus to persist.

4. Binding to Surface Immunoglobulin on B-Cells

Another mechanism some viruses use to evade the immune system is by binding to surface immunoglobulins (Ig) on B-cells. B-cells are responsible for producing antibodies that neutralize pathogens, including viruses. By binding to the surface of B-cells, viruses can effectively neutralize these immune cells, preventing them from producing antibodies and limiting the immune system’s ability to fight the infection.

This strategy has been observed in some retroviruses and RNA viruses, which can directly bind to B-cell receptors, reducing the effectiveness of the humoral immune response.

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Which Strategy Do Viruses Use to Evade the Immune System?

Among the options listed in the question, the correct answer is:

• (c) Virus-infected cells show reduced expression of surface MHC Class I molecules.

This strategy is widely used by viruses to evade immune detection. By reducing the presentation of viral peptides on MHC Class I molecules, infected cells are less likely to be recognized and attacked by cytotoxic T-cells. This evasion mechanism is one of the most common ways that viruses avoid immune surveillance and continue to replicate within the host.

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Conclusion

Viruses have evolved a variety of clever strategies to evade the human immune system, enabling them to persist and cause infections. By manipulating key immune components like MHC Class I molecules, T-cells, and B-cells, viruses can significantly reduce the effectiveness of the host’s immune response. Understanding these evasion mechanisms is crucial for developing better antiviral therapies and vaccines to combat viral infections and enhance immune defenses.

The immune system is continuously battling viruses, but the constant evolution of viral strategies makes this an ongoing challenge. Researchers are working tirelessly to uncover new ways to outsmart these pathogens and enhance our ability to fight them.