Final answer:
Macrophages fuse together in response to cytokines secreted by Th1 cells, which are part of the immune system's response to infection. These cytokines serve as signals to activate and recruit macrophages to sites where they can engulf and destroy pathogens.
Step-by-step explanation:
The signal that makes macrophages fuse together is largely attributed to cytokines secreted by T helper 1 (Th1) cells. These cytokines act as chemical messengers and play a pivotal role in the activation and recruitment of immune cells to sites of infection or inflammation. When Th1 cells secrete cytokines, they can activate macrophages and cytotoxic T cells to respond to pathogens. This process not only results in the activation of macrophages to destroy intracellular bacteria but also can lead to the accumulation and sometimes fusion of macrophages, forming structures known as giant cells or granulomas when the immune response is chronic.
Macrophages are derived from monocytes and are a key component of the immune system. They have the ability to move swiftly through the connective tissue matrix and engulf infectious agents and cellular debris. Macrophages can be roaming or free, moving by amoeboid movement, or fixed, serving as permanent residents within tissues. The macrophage's interaction with the adaptive immune system is facilitated through the presentation of antigens via the major histocompatibility complex (MHC) to T cells.
Additionally, macrophages and dendritic cells possess pattern recognition receptors (PRRs) which bind to pathogens and promote their engulfment and destruction. The process of signal integration occurs when signals from different cell-surface receptors combine to trigger the same response in a target cell, such as the activation of a macrophage.