Which of the following is a key function of class II gene products?

Class II gene products, which are primarily associated with the major histocompatibility complex (MHC), play a crucial role in the immune response by presenting antigens. Specifically, they are essential for presenting processed antigens to helper T cells (CD4+ T cells), which are involved in coordinating the immune response.

The key function of class II molecules is to display these antigens derived from extracellular sources. When a pathogen invades the body, antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells take up these antigens, process them, and then display the peptide fragments on their surface bound to MHC class II molecules. This presentation is crucial for the activation of helper T cells, which then enhance the immune response by stimulating the activity of B cells and cytotoxic T cells.

This is distinctly different from other functions mentioned in the options. For instance, while regulating blood pressure and initiating blood clotting are critical physiological processes, they do not directly involve class II gene products. Similarly, while producing antibodies is a function of B cells, it requires the activation and assistance of T helper cells that are, in turn, activated through the action of MHC class II molecules. Thus, the correct answer