Principles of Immunology
Organs of the Immune System
 Primary Lymphoid Organs
 Bone Marrow and Thymus
 Maturation Site
 Secondary Lymphoid Organs
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Spleen, lymph nodes,
MALT (mucosal associated lymph tissue)
GALT (gut associated lymph tissue)
Trap antigen, APC, Lymphocyte
Proliferation, provide site to interact
Lymphatic System
 Network of vessels
 Collects fluid from tissues, return to
blood
 Major cell is lymphocyte
 Unidirectional
 Often first place where antigens are
detected
1. Primary lymphoid organs
Thymus
 Bilobed Organ on Top of Heart
 Reaches Max. Size During Puberty
 70g infants, 3 g in adults
 95-99% Of T Cells Die in Thymus
 self reactivity or no reactivity to Ag
 Consists of Cortex and Medulla
 Rat Thymocytes
Thymus
 Function
 Takes in immature T cells and puts out
mature (immunocompetent) T cells
 Increased diversity of T cells
 T cell selection
Thymus
 T cell selection
 Based on MHC/Ag complex recognition
 Recognize MHC/Non self AG complexes
 Recognize MHC/Self Ag complexes
 Athymic condition
Bone marrow
 Structure
 Microscopic
 Less well defined than thymus
 Role of stromal cells
 Function
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Hematopoiesis
B cell maturation
B cell selection
Puts out mature, naive B cells
2. Secondary lymphoid organs
Lymph Nodes
 Structure
 Gross
 Bean-shaped structures
 Drains major segments of lymphatic system
Lymph Nodes
 Structure
 Microscopic
 Major cell types
 Lymphocytes
 Macrophages
 Dendritic cells
 Cortex/paracortex/medulla
 Follicles
 Primary
 Secondary
Lymph Nodes
 Function
 1st line of response to antigens
 Secondary follicle (Germinal center) is site of B
cell proliferation, mutation, differentiation
 Specificity is high
 >90% of B cells die through apoptosis
 After Ag stimualtion lymphocyte numbers up by
50X in efferent lymphatic vessel
 Lympadenopathy
Spleen
 Structure
 Gross
 Ovoid organ in upper left of abdomen
 Microscopic
 Compartmentalized
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Red pulp
White pulp
 Periarticualr lymphoid sheath
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Site of Ag presentation
 Major cell types
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Lymphocytes
Macrophages
Dendritic cells
RBCs
Spleen
 Function
 Filters out older RBCs
 Responds to Ag in circulatory system
 Produces activated B cells
 Splenectomy
Tonsils
 Follicular structure
 Contains lymphocytes, macrophages,
mast cells
 Germinal centers appear in response
to Ag
 Protective role in URI (Upper
respiratory Infection)
Appendix
 Associated with intestines
 Responds to Ag
 Role in GI immune response
MALT
 Lymphoid tissues below epithelium
 Presence of B cells
 Ag presented through unique cell (M
cell)
 Preferentially responds with IgA
antibody