Learning Objectives
By the end of this session, the learner will be able to describe the bidirectional interaction between macrophages and $T_h1$ cells, including the specific cytokines and surface molecules required for cellular immunity.
1. The Bidirectional Loop
The interaction between macrophages and T cells is not a one-way street; it is a positive feedback loop that amplifies the immune response against intracellular pathogens.
| Direction | Mechanism | Result |
|---|---|---|
Macrophage  T Cell |
Antigen presentation via MHC II to the TCR. | Activates the naive  T cell to become a  cell. |
| T Cell Macrophage |
Secretion of IFN- and CD40L expression. |
“Classical” activation (M1) of the macrophage. |
2. Two-Signal Activation of Macrophages
For a macrophage to become a potent killer of ingested microbes, it typically requires two specific signals from a
| Signal Type | Molecular Interaction | Functional Outcome |
|---|---|---|
| Cytokine Signal | IFN- (Interferon-gamma) |
Enhances phagocytosis and production of ROS/RNS. |
| Contact Signal | CD40L (on T cell) binds to CD40 (on Macrophage) | Triggers intracellular signaling for cytokine production and survival. |
3. Effector Functions of Activated Macrophages
Once activated by
| Function | Mechanism |
|---|---|
| Microbicidal Activity | Increased production of Reactive Oxygen Species (ROS) and Nitric Oxide (NO). |
| Inflammation | Secretion of pro-inflammatory cytokines like TNF- , IL-1, and IL-12. |
| Granuloma Formation | Chronic activation leads to “epithelioid” macrophages walling off pathogens (e.g., in TB). |
Clinical Pearls:
- Granulomatous Disease: If this interaction is defective (e.g., IFN-
- IL-12 Role: Macrophages secrete IL-12 to stimulate more T cells to become $T_h1$ cells, further driving the production of IFN-