Overview All white blood cells are nucleated, which distinguishes them from the anucleated red blood cells and platelets. Types of leukocytes can be classified in standard ways. Two pairs of broadest categories classify them either by structure (
granulocytes or
agranulocytes) or by cell lineage (myeloid cells or lymphoid cells). These broadest categories can be further divided into the five main types:
neutrophils,
eosinophils,
basophils,
lymphocytes, and
monocytes. These types are distinguished by their physical and functional characteristics. Monocytes and neutrophils are
phagocytic. Further subtypes can be classified. Granulocytes are distinguished from agranulocytes by their nucleus shape (lobed versus round, that is, polymorphonuclear versus mononuclear) and by their cytoplasm
granules (present or absent, or more precisely, visible on light microscopy or not thus visible). The other dichotomy is by lineage: Myeloid cells (neutrophils, monocytes, eosinophils and basophils) are distinguished from lymphoid cells (lymphocytes) by
hematopoietic lineage (
cellular differentiation lineage). Lymphocytes can be further classified as T cells, B cells, and natural killer cells.
Neutrophil Neutrophils are the most abundant white blood cell, constituting 60–70% of the circulating leukocytes. This gives the neutrophils the appearance of having multiple nuclei, hence the name polymorphonuclear leukocyte. The cytoplasm may look transparent because of fine granules that are pale lilac when stained. Neutrophils are active in phagocytosing bacteria and are present in large amount in the pus of wounds. These cells are not able to renew their
lysosomes (used in digesting microbes) and die after having phagocytosed a few pathogens. Neutrophils are the most common cell type seen in the early stages of acute inflammation. The average lifespan of inactivated human neutrophils in the circulation has been reported by different approaches to be between 5 and 135 hours.
Eosinophil Eosinophils compose about 2–4% of white blood cells in circulating blood. This count fluctuates throughout the day, seasonally, and during
menstruation. It rises in response to allergies, parasitic infections, collagen diseases, and disease of the spleen and central nervous system. They are rare in the blood, but numerous in the mucous membranes of the respiratory, digestive, and lower urinary tracts. They can be recognized by several coarse, dark violet granules, giving them a blue hue. The nucleus is bi- or tri-lobed, but it is hard to see because of the number of coarse granules that hide it. They secrete two chemicals that aid in the body's defenses:
histamine and
heparin. Histamine is responsible for widening blood vessels and increasing the flow of blood to injured tissue. It also makes blood vessels more permeable so neutrophils and clotting proteins can get into connective tissue more easily. Heparin is an anticoagulant that inhibits blood clotting and promotes the movement of white blood cells into an area. Basophils can also release chemical signals that attract eosinophils and neutrophils to an infection site.
Lymphocyte Lymphocytes are much more common in the lymphatic system than in blood. Lymphocytes are distinguished by having a deeply staining nucleus that may be eccentric in location, and a relatively small amount of cytoplasm. Lymphocytes include: •
B cells make
antibodies that can bind to
pathogens, block pathogen invasion, activate the
complement system, and enhance pathogen destruction. •
T cells: •
CD4+
T helper cells: T cells displaying
co-receptor CD4 are known as CD4+ T cells. These cells have
T-cell receptors and CD4 molecules that, in combination, bind
antigenic peptides presented on
major histocompatibility complex (MHC) class II molecules on
antigen-presenting cells. Helper T cells make
cytokines and perform other functions that help coordinate the
immune response. In
HIV infection, these T cells are the main index to identify the individual's immune system integrity. •
CD8+
cytotoxic T cells: T cells displaying co-receptor
CD8 are known as CD8+ T cells. These cells bind antigens presented on
MHC I complex of virus-infected or tumour cells and kill them. Nearly all nucleated cells display MHC I. •
γδ T cells possess an alternative
T cell receptor (different from the αβ TCR found on conventional CD4+ and CD8+ T cells). Found in tissue more commonly than in blood, γδ T cells share characteristics of helper T cells, cytotoxic T cells, and natural killer cells. •
Natural killer cells are able to kill cells of the body that do not display
MHC class I molecules, or display stress markers such as
MHC class I polypeptide–related sequence A (MIC-A). Decreased expression of MHC class I and up-regulation of MIC-A can happen when cells are infected by a virus or become cancerous.
Monocyte Monocytes, the largest type of white blood cell, share the "vacuum cleaner" (
phagocytosis) function of neutrophils, but are much longer lived as they have an extra role: they present pieces of
pathogens to T cells so that the pathogens may be recognized again and killed. This causes an antibody response to be mounted. Monocytes eventually leave the bloodstream and become tissue
macrophages, which remove dead cell debris as well as attack microorganisms. Neither dead cell debris nor attacking microorganisms can be dealt with effectively by the neutrophils. Unlike neutrophils, monocytes are able to replace their
lysosomal contents and are thought to have a much longer active life. They have the kidney-shaped nucleus and are typically not granulated. They also possess abundant cytoplasm. ==Fixed leucocytes==