3.2.3.1

Phagocytes

Phagocytes, are cells that ingest and destroy pathogens, a process called phago-

cytosis from the Greek “phagos” to eat. As shown in Figure 3.4, a foreign body is

internalized in a phagosome, which then fuses with the lysosome, resulting in de-

gradation of its contents. Phagocytes are distributed all over the body and can be

quickly recruited to the site of infection or injury. The examples of phagocytes are

neutrophils, macrophages, and dendritic cells.

a. Neutrophils

Neutrophils are specialized phagocytes that contain intracellular granules con-

taining several microbicidal molecules including reactive oxygen species, lyso-

zymes, and defensins. These molecules are released upon degranulation that is

triggered by a foreign organism. Neutrophils also attack microbes by extruding a

network of chromatin fibers containing antimicrobial peptides, thereby trapping and

neutralizing them [4].

b. Antigen-Presenting Cells

Antigen-presenting cells (APCs) such as macrophages and dendritic cells are

phagocytes that ingest foreign matter, and then chop the pathogen into small bits

which get displayed on their cell surface. By so doing, APCs display these frag-

ments to cells of the adaptive immune system, making a bridge between innate and

adaptive immunity.

FIGURE 3.4 Phagocytosis. A bacterium is engulfed by extensions formed by the phago-

cyte. It is internalized in a vacuole called the phagosome. The fusion of the phagosome with

the lysosome gives rise to the phagolysosome wherein the lysosomal enzymes degrade the

contents.

Introduction to basic immunology

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