Which component of the blood does NOT participate in the defense against antigens?

Erythrocytes, or red blood cells, primarily function in the transport of oxygen from the lungs to the tissues and the return of carbon dioxide from the tissues back to the lungs. They are not involved in the immune response or in the defense against antigens.

In contrast, other components of blood, such as white blood cells and lymphocytes, play crucial roles in identifying and combating pathogens. White blood cells encompass a variety of types that are integral to the immune system, including lymphocytes, which are specifically involved in recognizing and responding to antigens. Platelets, while primarily associated with blood clotting, also play a role in inflammation and the immune response, albeit indirectly.

Thus, erythrocytes do not participate in the defense against antigens and are focused solely on gas exchange in the body, making them the correct answer to this question.