Describe the sequence of events that occurs during a primary immune response with reference to the roles of: • macrophages • B-lymphocytes, including plasma cells • T-lymphocytes, limited to T-helper cells and T-killer cells

SouravOctober 30, 2024

Answer

The primary immune response is the body’s initial response to a specific pathogen after it is encountered for the first time. This response involves a complex interplay of various immune cells, including macrophages, B-lymphocytes (B-cells), and T-lymphocytes (T-cells). Here’s a detailed sequence of events that occur during a primary immune response:

1. Antigen Recognition and Phagocytosis

• Macrophages:
  • When a pathogen enters the body, macrophages act as antigen-presenting cells (APCs). They engulf (phagocytize) the pathogen through a process called phagocytosis.
  • After ingesting the pathogen, macrophages break it down into smaller pieces (antigens) and present these antigens on their surface using Major Histocompatibility Complex (MHC) class II molecules.

2. Activation of T-Lymphocytes

• T-helper Cells (CD4+ T-cells):
  • The antigen-presenting macrophages travel to the lymph nodes, where they interact with naive T-helper cells. The binding of the T-cell receptor (TCR) on T-helper cells to the antigen-MHC complex activates these T-cells.
  • Activated T-helper cells proliferate (multiply) and secrete cytokines, which play crucial roles in orchestrating the immune response by stimulating other immune cells, including B-cells and T-killer cells.

3. Activation of B-Lymphocytes

• B-Lymphocytes (B-cells):
  • Naive B-cells can also encounter the same antigen directly. When they bind to the antigen through their B-cell receptors (BCR), they become activated.
  • T-helper cells provide additional signals (via cytokines) that promote B-cell activation and differentiation.

4. Differentiation of B-Lymphocytes

• Activated B-cells undergo clonal expansion, where they proliferate and differentiate into two main types of cells:
  • Plasma Cells: These cells produce and secrete large amounts of antibodies (immunoglobulins) specific to the antigen. The antibodies circulate in the bloodstream and help neutralize or mark the pathogen for destruction.
  • Memory B-Cells: Some B-cells differentiate into memory B-cells, which remain in the body long-term and provide a quicker and stronger response if the same antigen is encountered again in the future.

5. Activation of T-Killer Cells

• T-Killer Cells (Cytotoxic T-Lymphocytes, CD8+ T-cells):
  • Some of the activated T-helper cells also stimulate naive T-killer cells. These cells recognize and bind to infected cells that present the specific antigen on MHC class I molecules.
  • Upon activation, T-killer cells proliferate and differentiate, seeking out and destroying infected cells by inducing apoptosis (programmed cell death).

6. Response and Resolution

• As the immune response progresses, antibodies produced by plasma cells help neutralize pathogens, while T-killer cells eliminate infected cells.
• Once the pathogen is cleared, the immune response begins to wind down. Memory B-cells and T-cells remain in circulation, providing long-lasting immunity.

Summary of Roles:

• Macrophages: Engulf pathogens, present antigens to T-cells, and secrete cytokines to recruit other immune cells.
• B-Lymphocytes: Produce antibodies to neutralize pathogens, differentiate into plasma cells (for immediate response) and memory B-cells (for long-term immunity).
• T-Lymphocytes:
  • T-Helper Cells: Activate and support B-cells and T-killer cells through cytokine secretion.
  • T-Killer Cells: Directly kill infected cells displaying the specific antigen.