The immune system is similar to a military where there are various branches, and each of them serves a dedicated function in order to protect the body. White blood cells are the first line of defense. They are first to identify pathogens and combat infection.
Lymphocytes, specific white blood cells function to enable the body to remember the microbes in case the body encounters in the future and fights them off faster. Bone marrow is an area where while blood cells are formed, lymph nodes form and store infection-fighting cells in the body, and lastly, there is spleen that controls the blood quantity in the body and removes the old or damaged cells.