The structure that helps certain bacteria ward off white blood cells is called a capsule.

Here's how it works:

* Capsule: A capsule is a thick, outer layer of polysaccharides (sugars) that surrounds the bacterial cell. This layer is often slimy and can be quite substantial.

* Evading Phagocytosis: White blood cells, particularly macrophages and neutrophils, engulf and destroy bacteria through a process called phagocytosis. The capsule acts as a barrier, preventing the white blood cells from recognizing and attaching to the bacterial cell surface.

* Mimicry: Some bacterial capsules can even mimic the host's own tissues, further confusing the immune system and allowing the bacteria to evade detection.

In summary: the capsule is a key adaptation that allows certain bacteria to escape the immune system's defenses.