Properties of Neutrophils

Neutrophils have their origin in multi-potential stem cells in the bone marrow. They differentiate in the marrow and are released in a mature form, containing a full complement of bactericidal agents. They are short-lived cells which constitute 30-70% of the circulating white blood cells (leukocytes).

During differentiation in the marrow (2-3 days) the nucleus of the cell becomes multilobed (hence the name polymorphonuclear leukocyte), cell division ceases, and mitochondria and endoplasm reticule disappear from the cytoplasm. At the same time the cell becomes motile and actively phagocytic. Cytoplasmic granules are formed from the Golgi apparatus. These granules are called lysosomal and contain the various bactericidal and digestive enzymes which can destroy bacterial cells after engulfment. The contents of lysosomal granules include lysozyme, cationic proteins, acid hydrolyses, protease's, peroxidase and lactoferrin. Neutrophils also contain large stores of glycogen. Since they derive most of their metabolic energy from glycolysis, they can function efficiently in anaerobic environments.

Some additional properties of neutrophils are:

-Only half the neutrophils in human circulation are detectable in the blood; the rest adhere to vessel walls.

-For every circulating neutrophils, approximately 100 near mature cells are held in reserve in the bone marrow pool.

-Once a neutrophils enters the tissues, intestinal tract or respiratory tract, it never returns to the circulation.

Properties of Macrophages

Macrophages (also called mononuclear phagocytes) also arise from bone marrow stem cells which give rise to monocytes which develop into monocytes that are released into the blood stream. Monocytes make up 3-7% of the circulating white blood cells. The monocytes is actively phagocytic and bactericidal. Within 2 days or so, the blood stream monocytes (sometimes called wondering macrophages) emigrate into the tissues where they settle down, enlarge and become fixed macrophages (tissue histiocytes), which also have phagocytic potential. Macrophages are more active in phagocytosis than monocytes and develop many more granules containing hydrolytic enzymes. New macrophages can develop by cell division under inflammatory stimuli, but most macrophages are matured blood monocytes.

The total pool of macrophages is referred to as the system of mononuclear phagocytes. The system is scattered throughout connective tissue, basement membranes of small blood vessels, liver sinusoids, spleen, lung , bone marrow and lymph nodes. Monocytes from the blood migrate into virtually every organ in the body where they mature into fixed macrophages. In the lymph nodes, macrophages function as scavengers to remove foreign material from the circulation.

Compared to neutrophils, macrophages are long-lived cells. As phagocytes, neutrophils play a more important role in the acute stages of an infection, while macrophages are principally involved in chronic types of infections. Neutrophils circulate in the blood stream, and during an acute inflammatory response they migrate through the endothelial cell junctions as part of the inflammatory exudate. They migrate to the focus of the infection and ingest or phagocytose foreign agents, Neutrophils which have become engorged with bacteria usually die and largely make up the material of pus. Macrophages, which are also attracted to the area during an inflammatory response, are slower to arrive and become increasingly involved in chronic infections. They, too, are actively phagocytic and will engulf and destroy foreign particles such as bacteria. However, macrophages have another indispensable function in host defense: they "process" the antigenic components of infective agents and present them to lymphocytes, a process that may usually required for the initiation of immunological  responses of the host. Macrophages and related dendritic cells are among an elite corps of antigen-presenting cells or APC's.

The Phagocytic Process

Phagocytosis and destruction of engulfed bacteria involves the following sequence of events:

1. Delivery of phagocytic cells to the site of infection

2. Phagocytic adherence to the target

3. Ingestion or engulfment of the target particle

4. Phagolysosome formation

5. Intracellular killing

6. Intracellular digestion (and egestion, in the case of macrophages)

These steps involved in the phagocytic process in macrophages are illustrated below.

Figure 6. Phagocytosis by a Macrophage. A bacterium, which may or may not be opsonized, is engulfed by the process of endocytosis. The bacterium is ingested in a membranous vesicle called the phagosome. Digestive granules (lysosomes) merge with phagosome, release their contents, and form a structure called the phagolysosome. The killing and digestion of the bacterial cell takes place in the phagolysosome. The macrophage egests debris while processing the antigenic components of the bacterium, which it returns to its surface in association with MHC II for antigen presentation to TH cells.

Diapedisis: the migration of cells across vascular walls which is initiated by the mediators of inflammation (kinins, histamine, prostaglandins, etc.)

Chemotaxis. Phagocytes are motile by ameboid action. Chemotaxis is movement of the cells in response to a chemical stimulus. The eventual concentration of phagocytes at a site of injury results from chemotactic response by the phagocytes which is analogous to bacterial chemotaxis. A number of chemotactic factors (attractants) have been identified, both for neutrophils and monocytes. These include bacterial products, cell and tissue debris, and components of the inflammatory exudate such as peptides derived from complement.

Phagocytic adherence

Phagocytosis is initiated by adherence of a particle to the surface of the plasma membrane of a phagocyte. This step usually involves several types of surface receptors on the phagocyte membrane. Three major receptors on phagocytes recognize the Fc portion of IgG: one is for monomeric IgG and the others are for antigen-crosslinked IgGs. Another receptor binds a complement factor C3b. Other phagocyte receptors bind fibronectin and mannose-terminated oligosaccharides. Under certain circumstances of infection, bacteria or viruses may become coated or otherwise display on their surfaces one or another of these substances (i.e., IgG, C3b, fibronectin or mannose). Such microbes are said to be opsonized and such substances as IgG or complement C3b bound to the surface of microbes are called opsonins. (Opsonin comes from a Greek word meaning "sauce" or "seasoning": they make the microbe more palatable and more easily ingested by the phagocyte.) Opsonins provide extrinsic ligands for specific receptors on the phagocyte membrane, which dramatically increases the rate of adherence and ingestion of the pathogen. Opsonized bacteria can be cleared from the blood by phagocytes; many types of non opsonized bacteria cannot be cleared.

Less firm attachments of a phagocyte to a particle can take place in the absence of opsonization. This can be thought of as nonspecific attachment which might be due to net surface charge on the phagocyte or particle and/or hydrophobicity of the particle.

Ingestion

After attachment of the phagocyte to its target, some sort of signal generation, which is poorly understood, results in physical or chemical changes in the cell that triggers ingestion. Ingestion is an engulfment process that involves infolding or invagination of the cell membrane enclosing the particle and ultimately releasing it into the cytoplasm of the cell within a membrane vesicle. The end result of ingestion is entry of the particle enclosed in a vesicle derived from the plasma membrane of the cell. This structure is called the phagosome.