0.1 Module 2a & 2b: barrier defenses and the innate immune  (Page 2/23)

Cells of the innate immune response

A phagocyte is a cell that is able to surround and ingest a particle or cell, a process called phagocytosis    . The phagocytes of the immune system engulf other particles or cells, either to clean an area of debris, old cells, or to kill pathogenic organisms such as bacteria. The phagocytes are the body’s fast acting, first line of immunological defense against organisms that have gotten through the barrier defenses and have entered the vulnerable tissues of the body.

Phagocytes: macrophages and neutrophils

Many of the cells of the immune system have a phagocytic ability, at least at some point during their life cycles. Phagocytosis is an important and effective mechanism of destroying pathogens during innate immune responses. The phagocyte takes the organism inside itself , effectively killing many pathogens. On the other hand, some bacteria including Mycobacteria tuberculosis , the cause of tuberculosis, may be resistant to these cells and are therefore much more difficult to clear from the body. Macrophages and neutrophils, are the major phagocytes of the immune system.

A macrophage    is an irregularly shaped phagocyte that is amoeboid (changes shape) and is the most useful of the phagocytes in the body. Macrophages move through tissues and squeeze through capillary walls. They participate in innate immune responses. Macrophages exist in many tissues of the body, either freely roaming through connective tissues or fixed to fibers within specific tissues such as lymph nodes. When pathogens breach the body’s barrier defenses, macrophages are the first line of defense ( [link] ).

A neutrophil    is a phagocytic cell that is attracted via chemical messenger from the bloodstream to infected tissues. Whereas macrophages act like sentries, always on guard against infection, neutrophils can be thought of as military reinforcements that are called into a battle to hasten the destruction of the enemy. The neutrophil is thought of as the primary pathogen-killing cell of the inflammatory process of the innate immune response.

A monocyte    is a circulating cell that changes into a macrophage which can be rapidly attracted to areas of infection by signal molecules of inflammation.

Natural killer cells

Natural killer (NK) cells are a type of lymphocyte that have the ability to induce apoptosis , that is, cell death, in cells infected with pathogens such as bacteria and viruses. NK cells can induce apoptosis, in which a series of events inside the cell which causes the cell to self destruct. The cell programs its own death by either of two mechanisms:

1) NK cells are able to respond to chemical signals. The chemical is a surface molecule that binds to the fas molecule on the surface of the infected cell, sending it apoptotic signals, thus killing the cell and the pathogen within it; or