What is a common function associated with blood cells?

Fighting infections is a vital function of certain types of blood cells, particularly white blood cells (leukocytes). These cells are key components of the immune system, playing a crucial role in defending the body against pathogens such as bacteria, viruses, and other foreign invaders. Different types of white blood cells have specific roles; for example, neutrophils are often the first responders to infection, while lymphocytes (like T-cells and B-cells) are important for recognizing and remembering pathogens for more effective responses in the future. This immune response is crucial for maintaining health and preventing disease.

In contrast, the other options do not align with the primary functions of blood cells. While blood does help transport nutrients and oxygen to tissues, this function is mainly attributed to red blood cells and plasma, not blood cells in general. Hormone production is primarily the function of endocrine glands rather than blood cells. Generating energy is more related to cellular metabolism and not a direct function of blood cells. Thus, the focus on blood cells specifically highlights their critical role in immune defense, making fighting infections the correct answer.