The correct answer is White blood cells.

White blood cells, also known as leukocytes, are a crucial component of the immune system. Unlike red blood cells, which are primarily involved in oxygen transport, white blood cells play a vital role in defending the body against infection and disease. A key characteristic of many types of white blood cells is their amoeboid movement. This means they can change their shape and extend projections called pseudopodia to move around. This ability is essential for their function, allowing them to squeeze through the narrow spaces between the cells that make up the capillary walls. This process is called diapedesis and is crucial for white blood cells to reach sites of infection or injury within the body's tissues.

Red blood cells lack the ability for amoeboid movement and are confined to the circulatory system. Guard cells, found in plants, regulate the opening and closing of stomata, and nerve cells transmit electrical signals throughout the nervous system; neither demonstrate amoeboid movement.

In summary, the unique amoeboid movement of white blood cells, facilitated by pseudopodia, enables them to navigate the circulatory system and migrate through capillary walls to reach infection sites, a process vital for immune response.