Question

What is one of the differences between \(\mathrm{CD} 4^{+}\) and \(\mathrm{CD} 8^{+}\) T cells? a. \(\mathrm{CD} 4^{+}\) cells are immature, and \(\mathrm{CD} 8^{+}\) cells are mature. b. \(\mathrm{CD} 4^{+}\) cells are activated, and \(\mathrm{CD} 8^{+}\) cells are not. c. \(\mathrm{CD} 4^{+}\) cells interact with class II MHC proteins, and \(\mathrm{CD} 8^{+}\) cells interact with class I MHC proteins. d. \(\mathrm{CD} 4^{+}\) cells activate cell-mediated responses, and \(\mathrm{CD} 8^{+}\) cells activate humoral responses.

Short answer

The correct answer is: \[ \textrm{CD} 4^{+} \textrm{cells interact with class II MHC proteins, and CD} 8^{+} \textrm{cells interact with class I MHC proteins}. \]

Step-by-step solution

Examine each option

To find the correct difference between CD4+ and CD8+ T cells, let's look at each of the choices and evaluate their validity.

Option A

The statement in option A claims that CD4+ cells are immature, while CD8+ cells are mature. This statement is untrue as both CD4+ and CD8+ T cells have developed into mature immune cells. They just have different roles in the immune system.

Option B

Option B states that CD4+ cells are activated and CD8+ cells are not activated. This statement is also incorrect because both CD4+ and CD8+ T cells can be activated. They are both capable of responding to specific pathogens in the body.

Option C

In option C, it is mentioned that CD4+ cells interact with class II MHC proteins and CD8+ cells interact with class I MHC proteins. This statement is accurate. CD4+ T cells, also known as helper T cells, recognize antigens presented on class II MHC proteins, while CD8+ T cells, also called cytotoxic T cells, recognize antigens presented on class I MHC proteins.

Option D

The statement in option D says that CD4+ cells activate cell-mediated responses, while CD8+ cells activate humoral responses. This statement is incorrect. CD4+ T cells actually help to regulate both cell-mediated (cytotoxic T cells) and humoral (B cell) immune responses, while CD8+ T cells directly kill infected target cells through a cell-mediated immune response.

Answer

The correct answer is option C: CD4+ cells interact with class II MHC proteins, and CD8+ cells interact with class I MHC proteins.

Key concepts

MHC Proteins

MHC proteins, or Major Histocompatibility Complex proteins, are crucial for the immune system's ability to detect and respond to pathogens. These proteins display fragments of pathogens (antigens) on the surface of cells. There are two main classes of MHC proteins: Class I and Class II.

Class I MHC proteins are found on almost all nucleated cells and present antigens from within the cell, such as those from a viral infection. In contrast, Class II MHC proteins are mainly on the surface of specialized immune cells known as antigen-presenting cells (APCs), like dendritic cells, macrophages, and B cells. These proteins present antigens that have been engulfed and processed by the APCs, indicating the presence of extracellular pathogens.

This distinction is fundamental in understanding how CD4+ and CD8+ T cells are different, as CD4+ T cells (helper T cells) typically interact with antigens presented by Class II MHC proteins, while CD8+ T cells (cytotoxic T cells) recognize those presented by Class I MHC proteins.

T Cell Activation

T cell activation is a critical step in the adaptive immune response and involves several key elements, including the T cell receptor (TCR), MHC proteins, and co-stimulatory signals.

When a T cell encounters an antigen presented by an MHC protein that fits its unique TCR, it becomes activated. However, for full activation, T cells also require secondary signals provided by co-stimulatory molecules on the APCs. CD4+ and CD8+ T cells are activated through these interactions, but they respond differently. CD4+ T cells, upon activation, become helper T cells that assist other immune cells, whereas CD8+ T cells become cytotoxic T cells that directly attack cells harboring intracellular pathogens.

Because both cell types require activation before they can perform their functions, it's clear that both CD4+ and CD8+ T cells, contrary to some misconceptions, can indeed be activated. It's their mode of action post-activation that differs, not their capacity to be activated.

Immune System Education

The process of immune system education is primarily centered around the development and maturation of T cells. This education occurs in the thymus, an organ where immature T cells, known as thymocytes, learn to distinguish between the body's own cells and foreign invaders.

T cells undergo positive selection, where they are 'taught' to recognize and interact with MHC proteins. This ensures that T cells will be able to recognize these proteins once they have exited the thymus. Following positive selection, T cells also undergo negative selection, which 'teaches' them not to react to self-antigens presented by MHC proteins, preventing autoimmunity.

Thus, both CD4+ and CD8+ T cells undergo an extensive and critical education process to ensure they function appropriately within the immune system. This education ensures that they are neither over-reactive, which could lead to autoimmune disorders, nor under-reactive, which could leave the body vulnerable to infections.

Cell-Mediated Immune Response

The cell-mediated immune response is a defense mechanism that does not involve antibodies, which distinguishes it from the humoral immune response. Instead, it relies on T cells to carry out its actions.

CD8+ T cells are central to this response; once activated, they become cytotoxic T cells capable of seeking out and destroying infected or cancerous cells. These T cells recognize infected cells when they present pieces of the intruder on their Class I MHC proteins and deliver lethal hits to eradicate them.

CD4+ T cells, while not directly killing infected cells, play a supportive role by secreting cytokines that regulate the activity of other immune cells, including CD8+ T cells and B cells. They can indirectly influence cell-mediated immunity by guiding and enhancing the cytotoxic functions of CD8+ T cells and bolstering the immune response.

Overall, cell-mediated responses are essential for tackling intracellular infections, such as those caused by viruses, and showcase the integrated roles CD4+ and CD8+ T cells play within the broader immune system.