Chapter 2

Chapter 2
Cells and Organs of the Immune System
Dr. Capers
Kindt • Goldsby • Osborne
Sixth Edition
Chapter 2:
Cells and Organs
of the Immune System
Copyright © 2007 by W. H. Freeman and Company
All blood cells arise from Hematopoietic
Stem Cells (HSC)
 Study of these cells is difficult
○ Scarce
○ Difficult to grow in vitro
Early in hematopoiesis, stem cell
differentiates to either
○ Lymphoid progenitor cell
○ Myeloid progenitor cell
- Progenitor cells have lost ability for self renewal
and are committed to particular cell lineage
Organized hierarchy
 Most of proliferation
takes place in
precursors (that are
NOT self-renewing)
rather than
hematopoietic stem
 Lowers chance of
Regulated at gene level
○ Transcription factors play important roles in
○ Studies using “knockout” mice
- Gene inactivated, if RBC or a particular WBC fails
to develop, it is concluded that protein was involved
in development of that cell
Hematopoietic Homeostasis
○ Average life span: 120 days
○ Phagocytosed by macrophages in spleen
○ Life spans from 1 day to 20-30 years
Apoptosis – programmed cell death
Normal WBC
WBC going through
Cells of the Immune System
○ 20-40% of WBC
○ 3 populations
- B cells
- T cells
- Natural Killer Cells
B cells and T cells
 Adaptive immunity
 Small lymphocytes
 Those that have not interacted with antigen
are called naïve
 Interaction with antigen – proliferation into
effector cells (i.e. plasma cells) and memory
B and T cells
B Lymphocytes (B cells)
 Site of maturation
○ Bursa of fabriscus in birds
○ Bone marrow in mammals
 Display membrane-bound immunoglobulin
 Once antigen is encountered:
 Differentiation
- Plasma cells – antibody can be secreted, die within 12 weeks
- Memory B cells – same membrane-bound antibody
as parent B cell, longer life span
T Lymphocytes (T cells)
 Site of maturation
○ Thymus
 T cell receptor
○ Only recognize antigen that is bound to cell membrane
proteins called major histocompatibility complex (MHC)
○ Once antigen in encountered with MHC:
 Differentiation
- Effector T cells
- Memory T cells
 2 subpopulations
 T helper (TH)
 T cytotoxic (TC)
 And now T regulatory (Treg)
T helper cells
○ CD4 glycoprotein
○ “help” activation of B cells, TC cells,
macrophages in immune response
T cytotoxic cells
 CD8 glycoprotein
 Recognition of MHC-antigen complex
initiates differentiation into effector cell called
cytotoxic T lymphocyte
 Eliminates infected cells or cancerous cells
T regulatory cells
 CD4 and CD25 glycoproteins
 Help suppress the immune system
Natural Killer Cells
 Innate immune response
 Large, granular
 Recognize tumor or virus-infected cells
 CD16 – which can recognize a region of
antibody that has attached to cell infected by
Other Leukocytes
Mononuclear phagocytes
 Monocytes circulate in blood and then migrate into
tissue and differentiate into specific macrophage
 Macrophages
Intestinal macrophages in gut
Alveolar macrophages in lung
Histiocytes in connective tissue
Kupffer cells in the liver
Mesangial cells in the kidney
Microglial cells in the brain
Osteoclasts in bone
 Activated macrophages are more effective than resting ones
Other Leukocytes
Mononuclear phagocytes
○ Complex antigens are phagocytized, the resulting
phagosome fuses with a lysosome
○ The digested antigen is then eliminated through
- Some of it is presented on membrane on MHC
○ Phagocytosis is enhanced when antibody is
attached to the antigen
- Antibody acts as opsonin: molecule that binds to both
antigen and phagocyte
Macrophage and bacteria
Other Leukocytes
○ Neutrophils
○ Eosinophils
○ Basophils
Other Leukocytes
Granulocytes – Neutrophils
 Multi-lobed nucleus, light granules
 1st to arrive at site of inflammation
 High #’s is 1st indication of infection
 Phagocytize
 Generate antimicrobial agents
Other Leukocytes
Granulocytes – Eosinophils
 Phagocytize
 Play a role in parasitic organisms
Other Leukocytes
Granulocytes – Basophils
 Nonphagocytic
 Play a role in allergic reactions
Other Leukocytes
Mast cells
 Play important role in development of
Other Leukocytes
Dendritic cells
 Long membranous extensions, look like
dendrites on nerve cells
 Antigen presentation
 4 major groups:
Langerhans DC
Interstitial DC
Monocyte-derived DC
Plasmacytoid-derived DC
Follicular dendritic cells
 Involved with B cell maturation
Organs of the Immune System
○ Thymus and bone marrow
○ Place of maturation of lymphocytes
○ Lymph nodes, spleen, mucosa-associated
lymphoid tissues such as gut-associated
lymphoid tissues
○ Mature lymphocytes interact with antigen
Primary Lymphoid Organs
Bone marrow
 Lymphocytes arise there, T cells go to
thymus to mature
 B cells mature here
 90% of plasma IgG and IgA comes from B
cells in the bone marrow
Primary Lymphoid Organs
○ T cell development and maturation
○ Bilobed organ above heart
- Surrounded by capsule and divided into lobules
- Outer part of lobule is cortex, inner is medulla
- Network of epithelial cells, dendritic cells, and
○ Thymus will induce death of those T cells that
- Recognize self-MHC molecules
- Those that interact with MHC molecules too strongly
(could produce autoimmune disorder)
○ Function decreases with age
Lymphatic System
Interstitial fluid (the portion that doesn’t
enter venous system) is returned to
circulatory system by lymphatic vessels
 Largest lymphatic vessel – thoracic duct
○ Enters left subclavian vein
○ Lymph from right arm and right side of head enters
through right lymphatic duct, drains into right
Antigen is carried by lymph to lymph nodes
Secondary Lymphoid Organs
Primary follicle
 Unactivated lymphoid
Secondary follicle
 Follicle that is
activated by antigen
 Ring of B cells that
surround germinal
 Proliferating B cells
and T helper cells
Secondary Lymphoid Organs
Lymph Nodes
 Encapsulated
 3 regions:
○ Cortex
 B cells, macrophages, dendritic cells
 Primary follicles
○ Paracortex
 T cells, dendritic cells
○ Medulla
 Plasma cells secreting antibody
Secondary Lymphoid Organs
 Filters blood, traps blood-bourne antigens
○ Important in systemic infections
 Blood enters through splenic artery
 Encapsulated
 Structure:
○ Projections from capsule form trabeculae
○ Compartments:
 Red pulp
- Macrophages, red blood cells
 White pulp
- Surrounds branches of splenic artery
- Forms PALS (periarteriolar lymphoid sheath)
- Primary follicles rich in B cells
Secondary Lymphoid Organs
Mucosa-Associated Lymphoid Tissue
 Organized areas along digestive, respiratory,
and urogenital tracts
 Very well organized areas in intestine are referred to
as Peyer’s patches
 Includes tonsils and appendix

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