Problem 7 - Campbell Biology

Question

Which of the following is least likely to help a virus avoid triggering an adaptive immune response? (A) having frequent mutations in genes for surface proteins (B) infecting cells that produce very few MHC molecules (C) producing proteins very similar to those of other viruses (D) infecting and killing helper T cells

Step-by-Step Solution

Answer

Option (C) producing proteins very similar to those of other viruses

Step 1: Understand the Adaptive Immune Response

The adaptive immune response is a specialized system that targets and eliminates specific pathogens. It includes the production of antibodies by B cells and the activation of T cells which recognize infected cells through MHC molecules displaying pathogenic antigens.

Step 2: Analyze Option (A)

Frequent mutations in genes for surface proteins can help a virus avoid detection by the immune system because the antibodies and T-cells may not recognize the mutated proteins, making this option likely to help avoid an immune response.

Step 3: Analyze Option (B)

Infecting cells that produce few MHC molecules can hinder the presentation of viral antigens to T cells, thus avoiding detection by the immune system, making this option also likely to help avoid an immune response.

Step 4: Analyze Option (C)

Producing proteins very similar to those of other viruses would not help avoid an immune response. If the immune system has encountered those other viruses before, it might recognize the proteins and mount a response.

Step 5: Analyze Option (D)

Infecting and killing helper T cells is a strategy used by some viruses, such as HIV, to impair the immune system’s ability to coordinate an effective response, making this option likely to help avoid an immune response.

Step 6: Compare All Options

Among the options, (A), (B), and (D) all help a virus in avoiding the adaptive immune response in different ways. Option (C) is unique because producing proteins similar to those of other viruses does not help in avoiding detection by the adaptive immune system.

Key Concepts

Virus Mutation

Viruses can evolve rapidly through mutations. Mutations are changes in the virus's genetic material, which can lead to alterations in its surface proteins. These proteins are often what the immune system recognizes and targets.
Frequent mutations create new variations of these proteins, making it difficult for the immune system to keep up. This is a survival strategy for viruses as antibodies and T cells may fail to recognize and attack the mutated proteins.

• The immune system might be prepared to fight an old version but not the new one.
• This constant change helps many viruses, like influenza, evade immune detection.

Sensitivity to mutations is one reason why new flu vaccines are needed annually. Each year's vaccine targets the most common strains, which have undergone several mutations compared to the previous year.

MHC Molecules

MHC stands for Major Histocompatibility Complex. These molecules are crucial for the immune system's ability to recognize infected cells.
MHC molecules display bits of pathogens (antigens) on the surfaces of infected cells. This 'presentation' is like showing a wanted poster to T cells.
If a virus infects cells that produce fewer MHC molecules, it becomes harder for T cells to 'see' the infected cells.

• MHC I molecules are present on almost all cells and signal to cytotoxic T cells.
• MHC II molecules are on specialized cells and activate helper T cells.

Reduced MHC expression means less antigen display and lower chances for immune cells to detect and destroy infected cells, helping the virus to persist longer.

Helper T Cells

Helper T cells play a vital role in shaping and guiding the adaptive immune response. They do not kill infected cells directly but instead 'help' other immune cells respond.
Helper T cells activate B cells to produce antibodies and cytotoxic T cells to kill infected cells. If a virus infects and kills these helper T cells, the entire adaptive immune response is crippled.

• No activation of B cells means no antibodies.
• No activation of cytotoxic T cells means infected cells are not destroyed.

A notable example is the HIV virus, which targets and destroys helper T cells, leading to a weakened immune system and increased susceptibility to infections.

Immune System Evasion

Pathogens like viruses have evolved numerous strategies to evade the immune system. Evasion ensures their survival and prolonged infection.
Some common strategies include:

• Frequent mutations (as discussed) that alter surface proteins and avoid immune detection.
• Infecting cells with low MHC molecule production, reducing antigen presentation.
• Mimicking host proteins to appear 'friendly' to the immune system.
• Directly attacking and crippling key immune cells like helper T cells.

These evasion techniques make it harder for the immune system to identify, target, and eliminate the virus.
Without effective recognition and response mechanisms, the virus can continue to replicate and spread within the host.