Problem 3 - Campbell Biology

Question

Which statement best describes the difference between responses of effector \(\mathrm{B}\) cells (plasma cells) and those of cytotoxic T cells? (A) \(\mathrm{B}\) cells confer active immunity; cytotoxic T cells confer passive immunity. (B) \(\mathrm{B}\) cells respond the first time a pathogen is present; cytotoxic T cells respond subsequent times. (C) \(\mathrm{B}\) cells secrete antibodies against a pathogen; cytotoxic T cells kill pathogen-infected host cells. (D) \(\mathrm{B}\) cells carry out the cell-mediated response; cytotoxic T cells carry out the humoral response.

Step-by-Step Solution

Answer

(C) B cells secrete antibodies against a pathogen; cytotoxic T cells kill pathogen-infected host cells.

Step 1: Understand B cells (plasma cells) response

B cells, also known as plasma cells when activated, play a crucial role in the immune system by producing and secreting antibodies specific to a pathogen.

Step 2: Understand cytotoxic T cells response

Cytotoxic T cells, also known as CD8+ T cells, are responsible for identifying and killing infected host cells that display a specific antigen on their surface.

Step 3: Analyze each statement

(A) B cells confer active immunity; cytotoxic T cells confer passive immunity: This is incorrect because both B cells and cytotoxic T cells mediate active immunity through different mechanisms. (B) B cells respond the first time a pathogen is present; cytotoxic T cells respond subsequent times: This is incorrect because both B cells and cytotoxic T cells are involved in primary and secondary immune responses. (C) B cells secrete antibodies against a pathogen; cytotoxic T cells kill pathogen-infected host cells: This is correct as it accurately describes the primary functions of B cells and cytotoxic T cells. (D) B cells carry out the cell-mediated response; cytotoxic T cells carry out the humoral response: This is incorrect because it reverses the roles; B cells are involved in the humoral response, while cytotoxic T cells perform cell-mediated responses.

Step 4: Select the best description

After analyzing all the options, the best description of the difference between responses of effector B cells and cytotoxic T cells is provided by statement (C): B cells secrete antibodies against a pathogen; cytotoxic T cells kill pathogen-infected host cells.

Key Concepts

B cells

B cells are a vital part of the adaptive immune system. These cells recognize specific pathogens through their unique receptors. Upon encountering a pathogen, B cells can differentiate into plasma cells. Plasma cells are essentially antibody factories.
They produce and secrete large quantities of antibodies that specifically target antigens presented by the pathogen. B cells are central to humoral immunity because of their role in producing these pathogen-specific antibodies.
Antibodies can neutralize pathogens directly, or mark them for destruction by other immune cells, making B cells crucial in defending the body against infections.

Cytotoxic T cells

Cytotoxic T cells, also known as CD8+ T cells, are key players in the cell-mediated immunity branch of the immune system. They specialize in identifying and destroying cells infected by viruses or other pathogens.
Once a cytotoxic T cell recognizes a pathogen-infected cell through antigen presentation, it releases chemicals like perforin and granzymes. These substances induce apoptosis, or programmed cell death, in the infected host cell.
Cytotoxic T cells are essential for controlling and clearing infections, as well as eliminating cancer cells by directly attacking compromised cells.

Antibodies

Antibodies are proteins produced by B cells and plasma cells in response to foreign antigens. These molecules are specifically designed to bind to particular antigens presented by pathogens such as bacteria or viruses.
The binding of antibodies to antigens can neutralize the pathogen directly or flag it for destruction by phagocytes or other immune cells.

• Neutralization: Antibodies can mask the infectivity of pathogens.
• Opsonization: Antibodies mark pathogens for phagocytosis.
• Complement Activation: Antibodies trigger the complement system leading to pathogen lysis.

Understanding antibodies and their functions is crucial for studying humoral immunity.

Cell-mediated immunity

Cell-mediated immunity is the immune response that does not involve antibodies but instead relies on T cells to combat pathogens.
In this pathway, cytotoxic T cells play a major role. They identify and destroy infected cells by recognizing small fragments of antigens presented on the surface of these cells.
The importance of cell-mediated immunity lies in its ability to target and eliminate cells that harbor intracellular pathogens like viruses, which cannot be effectively targeted by antibodies alone.

Humoral immunity

Humoral immunity refers to the immune response mediated by molecules found in body fluids or humors, primarily antibodies secreted by B cells and plasma cells.
This type of immunity is crucial for eliminating extracellular pathogens and toxins. Upon recognizing a pathogen, B cells get activated, proliferate, and differentiate into plasma cells, which then produce antibodies specific to that pathogen.
The antibodies circulate throughout the body fluids, neutralizing pathogens, facilitating their phagocytosis, or activating other immune response components.

• Neutralization and Agglutination: Antibodies can render pathogens non-functional or clump them together.
• Complement Activation: They activate the complement system leading to pathogen lysis.

This system is essential for systemic protection against infections.