Question

Which of these statements about class-2 MHC molecules is false? a. They are found on the surface of B lymphocytes. b. They are found on the surface of macrophages. c. They are required for B cell activation by a foreign antigen. d. They are needed for interaction of helper and cytotoxic T cells. e. They are presented together with foreign antigens by macrophages. In questions \(15-17\), match the cytokine with its description.

Short answer

Statement d is false, as class-1 MHC molecules are involved in the interaction between helper T cells and cytotoxic T cells, not class-2 MHC molecules.

Step-by-step solution

Understanding class-2 MHC molecules

Class-2 MHC (Major Histocompatibility Complex) molecules are cell surface glycoproteins involved in the immune system. They primarily function to present processed antigenic peptides to helper T lymphocytes (CD4+ T cells), initiating an immune response.

Analyzing statement a

Statement a states that class-2 MHC molecules are found on the surface of B lymphocytes. This is a true statement, as these proteins are present on the surface of B lymphocytes, which are antigen-presenting cells (APCs).

Analyzing statement b

Statement b states that class-2 MHC molecules are found on the surface of macrophages. This is also true, as macrophages are another type of antigen-presenting cells (APCs) that display class-2 MHC molecules on their surface.

Analyzing statement c

Statement c states that class-2 MHC molecules are required for B cell activation by a foreign antigen. This is true, as class-2 MHC molecules on the surface of B lymphocytes play a critical role in the activation of these cells by presenting antigenic peptides to helper T cells.

Analyzing statement d

Statement d says they are needed for the interaction of helper and cytotoxic T cells. This statement is false, as class-1 MHC molecules are involved in the interaction between helper T cells and cytotoxic T cells. Class-2 MHC molecules are only required for the activation of helper T cells through APCs.

Analyzing statement e

Statement e states that class-2 MHC molecules are presented together with foreign antigens by macrophages. This is true, as macrophages engulf and process the foreign antigens and then present the antigenic peptides on their surface complexed with class-2 MHC molecules for recognition by helper T cells. In conclusion, statement d is the false statement about class-2 MHC molecules.

Key concepts

B lymphocytes

B lymphocytes, or B cells, play a critical role in the adaptive immune response. These cells are a type of white blood cell and are responsible for producing antibodies. These antibodies are proteins that bind specifically to antigens, neutralizing them or marking them for destruction by other cells in the immune system.
B lymphocytes are unique because, unlike some other immune cells, they have specific receptors on their surface that recognize and attach to unique antigens. This specificity is vital for the immune system's ability to target and eliminate specific pathogens.

• They originate in the bone marrow, which is also why they are called 'B' cells.
• Once they encounter their specific antigen, B lymphocytes can differentiate into plasma cells that produce antibodies.
• B lymphocytes can also become memory cells, which "remember" the antigen and mount a quicker response upon future exposures.

In this way, B cells can not only fight off an infection they encounter, but also prepare the body for future defense against the same pathogen.

Antigen-presenting cells

Antigen-presenting cells (APCs) are essential mediators in the immune system. Their main job is to capture antigens, process them, and present them on their surface for recognition by T cells. This process is key in initiating an effective immune response.
There are three main types of antigen-presenting cells:

• Dendritic cells: Considered the most potent APCs, they initiate primary immune response by migrating to lymph nodes to present antigens to T cells.
• Macrophages: These are large phagocytic cells that engulf antigens and present them using class-2 MHC molecules.
• B lymphocytes: While primarily known for producing antibodies, they also act as APCs, displaying antigens on their class-2 MHC molecules to helper T cells.

This antigen presentation is critical for T cell activation, which in turn directs the immune response to target and eliminate pathogens.

Immune response

The immune response is a complex process that protects the body from infections and disease. It involves multiple types of cells and proteins that work together to recognize and eliminate foreign threats.
The immune response is divided into two main types: innate and adaptive.

• Innate immune response: This is the body's first line of defense and involves barriers like skin and mucous membranes, as well as phagocytic cells like macrophages.
• Adaptive immune response: More specialized, this involves lymphocytes such as B and T cells. These cells remember specific pathogens and respond more effectively on subsequent encounters.

During the adaptive response, B lymphocytes and T cells are activated by antigen presentation. Helper T cells (CD4+) recognize antigens presented by class-2 MHC molecules on APCs and orchestrate the immune response, while cytotoxic T cells (CD8+) target and kill infected cells. This coordinated response ensures that pathogens are efficiently eradicated.