Here's why:

* Fighting infections: White blood cells are the body's primary defense against bacteria, viruses, fungi, and parasites. They identify and destroy these invaders, preventing them from causing harm.

* Immune memory: Some white blood cells (like memory B cells) retain a "memory" of previous infections, allowing the body to mount a quicker and more effective response upon re-exposure.

* Inflammation: White blood cells are involved in the inflammatory response, which helps to isolate and destroy pathogens and repair damaged tissue.

While white blood cells are a vital part of the immune system, they are not the only component. Other important elements include:

* Lymphatic system: This network of vessels and organs helps to circulate white blood cells and filter pathogens.

* Antibodies: These proteins are produced by white blood cells and can bind to pathogens, neutralizing them.

* Cytokines: These signaling molecules help coordinate the immune response.