Which of the following help to protect the body from infectious disease?

Leukocytes, often referred to as white blood cells, play a crucial role in the body's immune system, which is essential for protecting against infectious diseases. These cells are part of the immune response and are responsible for identifying and attacking pathogens such as bacteria, viruses, and other foreign invaders. There are various types of leukocytes, each with specific roles in the immune process, including phagocytosis (the ingestion of harmful microorganisms) and the production of antibodies.

The formation of a robust immune response is vital in preventing infections and ensuring overall health. While erythrocytes (red blood cells) are responsible for transporting oxygen throughout the body, and thrombocytes (platelets) aid in blood clotting, they do not contribute to the immune defense against infections. Hematocyte, which generally refers to blood cells, is not a specific term used to describe a particular type of cell involved in immunity.

Thus, leukocytes are the primary defenders against infectious agents, making this choice the correct answer.