Defense Mechanisms of the Body
649
Table 18–1
Cells Mediating Immune Defenses
Name
Site Produced
Functions
Leukocytes (white blood cells)
Neutrophils
Bone marrow
1. Phagocytosis
2. Release chemicals involved in infl ammation (vasodilators,
chemotaxins, etc.)
Basophils
Bone marrow
Carry out functions in blood similar to those of mast cells in
tissues (see below)
Eosinophils
Bone marrow
1. Destroy multicellular parasites
2. Participate in immediate hypersensitivity reactions
Monocytes
Bone marrow
1. Carry out functions in blood similar to those of macrophages in
tissues (see below)
2. Enter tissues and transform into macrophages
Lymphocytes
Mature in bone marrow
(B cells and NK cells)
and thymus (T cells);
activated in peripheral
lymphoid organs
Serve as recognition cells in specifi c immune responses and are
essential for all aspects of these responses
B cells
1. Initiate antibody-mediated immune responses by binding
specifi c antigens to the B cell’s plasma membrane receptors,
which are immunoglobulins
2. During activation are transformed into plasma cells, which
secrete antibodies
3. Present antigen to helper T cells
Cytotoxic T cells (CD8 cells)
Bind to antigens on plasma membrane of target cells (virus-
infected cells, cancer cells, and tissue transplants) and directly
destroy the cells
Helper T cells (CD4 cells)
Secrete cytokines that help to activate B cells, cytotoxic T cells,
NK cells, and macrophages
NK cells
1. Bind directly and nonspecifi cally to virus-infected cells and
cancer cells and kill them
2. Function as killer cells in antibody-dependent cellular
cytotoxicity (ADCC)
Plasma cells
Peripheral lymphoid organs;
differentiate from B cells
during immune responses
Secrete antibodies
Macrophages
Bone marrow; reside in
almost all tissues and organs;
differentiate from monocytes
1. Phagocytosis
2. Extracellular killing via secretion of toxic chemicals
3. Process and present antigens to helper T cells
4. Secrete cytokines involved in infl
ammation, activation and
differentiation of helper T cells, and systemic responses to
infection or injury (the acute phase response)
Dendritic (macrophage-like) cells
Almost all tissues and
organs; microglia in the
central nervous system
Same as macrophages
Mast cells
Bone marrow; reside in
almost all tissues and organs;
differentiate from bone
marrow cells
Release histamine and other chemicals involved in infl
ammation
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