The correct answer is option 1: white blood cells act as a defense against infection.

White blood cells, also known as leukocytes, play a crucial role in our immune system. They are responsible for recognizing and fighting against harmful pathogens, such as bacteria, viruses, and fungi.

When the body is infected, white blood cells are recruited to the site of infection to destroy the pathogens and prevent further spread of the infection. They do this by engulfing and digesting the pathogens, producing antibodies to neutralize them, or releasing chemicals that can kill the invaders.

White blood cells also play a role in inflammation, which is part of the immune response. They help to remove dead cells and debris from the site of infection or injury, promoting healing and tissue repair.

In summary, white blood cells are an essential part of our immune system and act as our body`s defense mechanism against infections.