Lymphocytes recognize foreign antigens because of the _________ ________ on their cell membranes.

Lymphocytes are a type of white blood cell that play a crucial role in the immune response by identifying and responding to foreign substances, known as antigens. The ability of lymphocytes to recognize these antigens is primarily due to the presence of antigen receptors on their cell membranes. Antigen receptors are specialized proteins that are uniquely structured to bind to specific antigens, allowing lymphocytes to "see" and respond to pathogens such as bacteria or viruses.

Each lymphocyte expresses a unique set of antigen receptors that have been developed through a complex process of genetic rearrangement, which enables the immune system to recognize a vast array of foreign elements. When an antigen binds to its corresponding receptor on a lymphocyte, it triggers a cascade of immune responses, activating the lymphocyte and mobilizing other components of the immune system to combat the perceived threat.

The other options do not specifically refer to the mechanism by which lymphocytes recognize foreign antigens. While signal proteins and receptor proteins can play roles in cellular communication and signaling pathways, they do not specifically highlight the unique aspect of antigen recognition that is critical for lymphocyte function. Cell markers also refer to broader identifiers on cells, not necessarily tied to the specialized function of recognizing antigens. Therefore, “antigen receptors”