Final answer:
B cells are responsible for recognizing antigens directly and interacting with helper T cells through CD40 ligand-CD40 interactions in the formation of granulomas in response to Mycobacterium infection. These B cells act as antigen-presenting cells, presenting the antigen to T cells and initiating a cascade of immune responses.
Step-by-step explanation:
In the formation of granulomas in response to Mycobacterium infection, the cells that recognize antigens directly and interact with helper T cells through CD40 ligand-CD40 interactions are B cells. These interactions are vital for the adaptive immune response, particularly in the context of forming granulomas, which are a hallmark of chronic inflammation during infections such as tuberculosis. The B cells function as antigen-presenting cells (APCs), presenting the antigens to helper T cells, which is crucial for the subsequent amplification of the immune response. Macrophages and dendritic cells are also involved as APCs but the specific interaction mentioned—the CD40 ligand-CD40 interaction—is more characteristic of B cell helper T cell communication.
T cells are activated when a part of the pathogen, presented by B cells or other APCs on their Major Histocompatibility Complex (MHC) proteins, is recognized by the receptor on the T cell. This stimulates the T cells to proliferate and release cytokines, enhancing the immune response against the intracellular pathogen. In the process of granuloma formation during Mycobacterium infection such as tuberculosis, it is the interaction between B cells and helper T cells that is particularly important in coordinating the immune defense.