What are the general characteristics of MHC class II molecules?

MHC class II molecules are primarily involved in the immune response to extracellular pathogens, particularly bacteria and parasites. They are expressed mainly on professional antigen-presenting cells, such as dendritic cells, macrophages, and B cells. When these cells capture and process antigens from extracellular sources, they present these antigens in association with MHC class II molecules to CD4+ T helper cells. This interaction is crucial for activating T cells and for orchestrating a robust immune response against these types of infections.

The characteristic of being involved in the response to bacteria and parasites highlights the role of MHC class II molecules in mediating T cell help for antibody production and activating other immune cells that can combat these pathogens effectively. As a result, this makes option B the correct choice, as it encompasses the primary function of MHC class II in the context of the immune system.

In contrast, the other options do not accurately represent the characteristics of MHC class II molecules. For example, MHC class II molecules are not recognized by all nucleated cells; their expression is restricted to antigen-presenting cells. They also do not exclusively recognize viral antigens, as their main role is in presenting a variety of antigens from extracellular sources. Additionally, while they are