Chapter 2
Cells and Organs of the Immune System
Dr. Capers
Kindt • Goldsby • Osborne
Kuby IMMUNOLOGY
Sixth Edition
Chapter 2:
Cells and Organs
of the Immune System
Copyright © 2007 by W. H. Freeman and Company
Hematopoiesis

All blood cells arise from Hematopoietic
Stem Cells (HSC)
 Study of these cells is difficult
○ Scarce
○ Difficult to grow in vitro
Hematopoiesis

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
differentiated
precursors (that are
NOT self-renewing)
rather than
hematopoietic stem
cell
 Lowers chance of
cancer
Hematopoiesis

Regulated at gene level
○ Transcription factors play important roles in
hematopoiesis
○ 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

Erythrocyte
○ Average life span: 120 days
○ Phagocytosed by macrophages in spleen

WBC - LEUKOCYTES
○ Life spans from 1 day to 20-30 years

Apoptosis – programmed cell death
Normal WBC
WBC going through
apoptosis
Cells of the Immune System

Lymphocytes
○ 20-40% of WBC
○ 3 populations
- B cells
- T cells
- Natural Killer Cells
Lymphocytes

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
cells
Lymphocytes

B and T cells
Lymphocytes

B Lymphocytes (B cells)
 Site of maturation
○ Bursa of fabriscus in birds
○ Bone marrow in mammals
 Display membrane-bound immunoglobulin
(antibody)
 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
Lymphocytes

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)
Lymphocytes

T helper cells
○ CD4 glycoprotein
○ “help” activation of B cells, TC cells,
macrophages in immune response
Lymphocytes

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
Lymphocytes

T regulatory cells
 CD4 and CD25 glycoproteins
 Help suppress the immune system
Lymphocytes

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
virus
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
exocytosis
- 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

Granulocytes
○ 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
allergies
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

Primary
○ Thymus and bone marrow
○ Place of maturation of lymphocytes

Secondary
○ 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

Thymus
○ 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
macrophages
○ Thymus will induce death of those T cells that
can’t:
- 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
subclavian

Antigen is carried by lymph to lymph nodes
Secondary Lymphoid Organs

Primary follicle
 Unactivated lymphoid
follicle

Secondary follicle
 Follicle that is
activated by antigen
 Ring of B cells that
surround germinal
center
 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

Spleen
 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
 MALT
 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