Branched Cells That Gather Antigen From Tissues Bring To Lymphctyes

Lobed nucleus; granules in cytoplasm; ameboid appearance

Large eosinophilic granules; non-segmented or bilobed nucleus

Lobed nucleus; large basophilic granules

Single nucleus; abundant cytoplasm

Single nucleus, abundant cytoplasm

Branched

Single nucleus; little cytoplasm before differentiation

Account for most of the circulating leukocytes; few in tissues except during inflammation and in reserve locations

Few in tissues except in certain types of inflammation and allergies

Basophils in circulation; mast cells present in most tissues

In circulation; they differentiate into either macrophages or dendritic cells when they migrate into tissue

Present in virtually all tissues; given various names based on the tissue in which they are found

Initially in tissues, but they migrate to lymph nodes and other secondary lymphoid organs

In lymphoid organs (such as lymph nodes, spleen, thymus, appendix, tonsils); also in circulation

Phagocytize and digest engulfed materials

Participate in inflammatory reaction and immunity to some parasites

Release histamine and other inflammation-causing chemicals from the granules

Phagocytize and digest engulfed materials

Phagocytize and digest engulfed materials

Gather antigen from the tissues and then present it to the lymphocytes that congregate in the secondary lymphoid organs

Participate in adaptive immune responses virtually all tissues, they are particularly abundant in liver, spleen, lymph nodes, lungs, and the peritoneal (abdominal) cavity; they are given various names based on the tissue in which they are found. This widespread collection of phagocytic cells, along with monocytes, constitutes the mononuclear phagocyte system (MPS), formerly called the reticuloendothelial system (RES) (figure 15.5). The role of macrophages in phagocytosis and other aspects of host defense will be discussed in more detail later in the chapter. ■ specialized attributes of macrophages, p. 385 ■ Dendritic cells are mobile, branched cells that are highly phagocytic early in their life. They are intimately involved in adaptive immunity, functioning as scouts in various tissues throughout the body. Initially, they continually engulf antigens, gathering them from the tissues; eventually, however, they migrate to lymph nodes and other secondary lymphoid organs, which are regions where various cells of the immune system congregate. There they show fragments of the proteins they have collected to lymphocytes, a process called antigen presentation. Details of antigen presentation by dendritic cells with be discussed in chapter 16. ■ the role of dendritic cells in T-cell activation, p. 410 ■ secondary lymphoid organs, p. 396

Lymphocytes

Lymphocytes are involved in adaptive immunity. In contrast to the generic pattern recognition of antigens by cells of the innate defenses, individual cells of the two major groups of lymphocytes, B cells and T cells, show remarkable molecular specificity in their recognition of antigen. Relatively few of the millions of different B and T cells can respond to a given antigen. When these cells encounter that antigen, they must multiply in order to amass sufficient numbers of cells to mount an effective response. The role of lymphocytes is the primary topic in chapter 16. Characteristics of lymphocytes are described here:

■ B cells are responsible for producing antibodies.

Alveolar macrophages in lungs

Liver Kupffer cells

Liver Kupffer cells

Brain microglial cells

Lymph nodes

Splenic macrophages and blood monocytes

Brain microglial cells

Lymph nodes

Abdominal cavity (peritoneum) with peritoneal macrophages

Splenic macrophages and blood monocytes

Kidney mesangial phagocytes

Lymph node resident and recirculating macrophages

Precursors in bone marrow

Figure 15.5 Mononuclear Phagocyte System This system of monocytes and macrophages was formerly known as the reticuloendothelial system. Many of these cells have special names to denote their location—for example, Kupffer cells (in the liver) and alveolar macrophages (in the lung).

■ T cells can be divided into two main functional types. T-cytotoxic cells are responsible for destroying infected or abnormal host cells; T-helper cells coordinate the immune response.

■ Natural killer (NK) cells kill cells, but they do not specifically recognize that the target is an invader; instead, NK cells destroy cells that have been bound by antibody or that exhibit certain abnormal traits.

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