31/10/2020
Despite their differences in appearance, all of the various types of WBCs have a role in the immune response. They circulate in the blood until they receive a signal that a part of the body is damaged. Signals include interleukin 1 (IL-1), a molecule secreted by macrophages that contributes to the fever of infections, and histamine, which is released by circulating basophils and tissue mast cells, and contributes to allergic reactions. In response to these signals, the WBCs leave the blood vessel by squeezing through holes in the blood vessel wall. They migrate to the source of the signal and help begin the healing process.
Individuals who have low levels of WBCs may have more and worse infections. Depending upon which WBCs are missing, the patient is at risk for different types of infection. For example, macrophages are especially good at swallowing bacteria, and a deficiency in macrophages leads to recurrent bacterial infections. In contrast, T cells are particularly skilled in fighting viral infections, and a loss of their function results in an increased susceptibility to viral infections.
Neutrophils are also known as polymorphonuclear cells because they contain a nucleus whose shape (morph) is irregular and contains many (poly) lobes. They also belong to a group of WBCs known as granulocytes because their cytoplasm is dotted with granules that contain enzymes that helps them digest pathogens.
Monocytes are young WBCs that circulate in the blood. They develop into macrophages after they have left the blood and migrated into tissue. There they provide an immediate defense because they can engulf (phagocytose) and digest pathogens before other types of WBCs reach the area.
In the liver, tissue macrophages are called Kupffer cells, and they specialize in removing harmful agents from blood that has left the gut. Alveolar macrophages are in the lungs and remove harmful agents that may have been inhaled. Macrophages in the spleen remove old or damaged red blood cells and platelets from the circulation.
Lymphocytes are round cells that contain a single, large round nucleus. There are two main classes of cells, the B cells that mature in the bone marrow, and the T cells that mature in the thymus gland.
Reference
Dean L. Bethesda (MD): National Center for Biotechnology Information (US) ; 2005.