White blood cells are a collection of different immune cell types that respond to inflammation and directly attack invading pathogens during an innate immune response.
The key cellular players of the immune response are the leukocytes, or white blood cells, that circulate in the blood and lymph and search through tissues looking for any signs of pathogens or foreign particles. They are actively drawn toward pathogens during an inflammatory response through the process of chemotaxis, the movement of a cell in response to a chemical stimulus. Once they arrive at the site of infection, they have several tools that enable them to fight parasites, the most prominent of which is phagocytosis.
Phagocytosis is the process by which a leukocyte engulfs and destroys its target through a series of steps. The first step is contact, where the leukocyte uses its cell surface receptors to bind to the target, leading to the subsequent steps of phagocytosis. The process of phagocytosis is aided by antibodies and the complement system, which opsonize the target. The second step is engulfment, where the leukocyte changes the shape of its cell membrane to produce extensions in order to surround the target and engulf it entirely. Once the target is fully engulfed, it is internalized in a vesicle, a membrane sac formed from the plasma membrane, called a phagosome in the third step. In the fourth step, the target-containing phagosome then fuses with another type of vesicle in the cell, called a lysosome, an organelle in the cytoplasm of eukaryotic cells containing degradative enzymes enclosed in a membrane. The fusion of the two vesicles, phagosome and lysosome, results in the formation of a phagolysosome. In the final step, the target is completely degraded and killed using the contents of the lysosome.
While all leukocytes serve the same general function of fighting pathogens, some leukocytes are better at certain functions and situations than others. The neutrophil is the most common type of leukocyte in the body; it migrates from the bloodstream to carry out phagocytosis at the site of an injury. Neutrophils make up 40–60% of the leukocytes in the body. Neutrophils are typically phagocytic cells and respond to most types of infection. A basophil is a type of white blood cell that releases histamine and heparin to promote inflammatory reactions such as in allergic reactions, anaphylaxis, and asthma. Basophils make both histamine and serotonin, signalling molecules that act outside of the nervous system to induce inflammation responses. They are identified by their ability to absorb blue stain. An eosinophil is a leukocyte that contains granules and releases cytotoxic chemicals to kill large parasites. Eosinophils also aid in the regulation of allergies and asthma. A lymphocyte is one of a family of several different types of leukocytes, including B cells, T cells, and natural killer (NK) cells. Lymphocytes are found in circulation and lymphoid organs, such as lymph nodes. Monocytes are leukocytes that circulate in blood for only days before migrating into tissues where they differentiate to become macrophages and dendritic cells. Macrophages and dendritic cells constantly patrol for pathogens that they phagocytose and perform essential intermediary signalling functions between the innate and adaptive immune systems.