White blood cells (WBC) are a diverse collection of nucleated cells that can be found in the bloodstream throughout at least part of their lives. Their usual blood concentration ranges between 4000 and 10,000 microliters. Each milliliter of blood contains about 6 to 7 million WBCs. The main type of WBC is the neutrophil; it makes up approximately 80% of all WBCs. The remaining 20% are made up of lymphocytes, monocytes, eosinophils, and basophils. Neutrophils are the most common type of white blood cell. They are the most abundant type of white blood cell in our bodies and play an important role in protecting us from infection. There are several different types of neutrophils: segmented, banded, metamyelocyte, and spherical. Segmented neutrophils have three distinct segments or lobes, while banded neutrophils have two clear regions separated by a dark region. Metamyelocytes have one large nucleus with many threads attached to it, and spherical neutrophils do not have any visible nuclei. Lymphocytes are the second most common type of white blood cell. They are responsible for mounting immune responses against foreign organisms like bacteria and viruses. There are two major types of lymphocytes: B cells and T cells.