This document summarizes the primary and secondary lymphoid organs of the immune system. The primary lymphoid organs, the bone marrow and thymus, are sites of lymphocyte maturation. The secondary lymphoid organs, including the spleen, lymph nodes, and mucosal-associated lymphoid tissue (MALT), trap antigens and allow interactions between antigen-presenting cells and lymphocytes to trigger immune responses. The lymphatic system is a network of vessels that drains fluid from tissues and returns it to the blood, transporting lymphocytes between lymphoid organs.

1. Principles of Immunology
Organs of the Immune System

2.  Primary Lymphoid Organs
 Bone Marrow and Thymus
 Maturation Site
 Secondary Lymphoid Organs
 Spleen, lymph nodes,
 MALT (mucosal associated lymph tissue)
 GALT (gut associated lymph tissue)
 Trap antigen, APC, Lymphocyte
Proliferation, provide site to interact

3. Lymphatic System
 Network of vessels
 Collects fluid from tissues, return to
blood
 Major cell is lymphocyte
 Unidirectional
 Often first place where antigens are
detected

5. 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

7. Thymus
 Function
 Takes in immature T cells and puts out
mature (immunocompetent) T cells
 Increased diversity of T cells
 T cell selection

8. Thymus
 T cell selection
 Based on MHC/Ag complex recognition
 Recognize MHC/Non self AG complexes
 Recognize MHC/Self Ag complexes
 Athymic condition

9. Bone marrow
 Structure
 Microscopic
 Less well defined than thymus
 Role of stromal cells
 Function
 Hematopoiesis
 B cell maturation
 B cell selection
 Puts out mature, naive B cells

10. 2. Secondary lymphoid organs
Lymph Nodes
 Structure
 Gross
 Bean-shaped structures
 Drains major segments of lymphatic system

11. Lymph Nodes
 Structure
 Microscopic
 Major cell types
 Lymphocytes
 Macrophages
 Dendritic cells
 Cortex/paracortex/medulla
 Follicles
 Primary
 Secondary

15. 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

16. Spleen
 Structure
 Gross
 Ovoid organ in upper left of abdomen
 Microscopic
 Compartmentalized
 Red pulp
 White pulp
 Periarticualr lymphoid sheath
 Site of Ag presentation
 Major cell types
 Lymphocytes
 Macrophages
 Dendritic cells
 RBCs

18. Spleen
 Function
 Filters out older RBCs
 Responds to Ag in circulatory system
 Produces activated B cells
 Splenectomy

19. Tonsils
 Follicular structure
 Contains lymphocytes, macrophages,
mast cells
 Germinal centers appear in response
to Ag
 Protective role in URI (Upper
respiratory Infection)

20. Appendix
 Associated with intestines
 Responds to Ag
 Role in GI immune response

21. MALT
 Lymphoid tissues below epithelium
 Presence of B cells
 Ag presented through unique cell (M
cell)
 Preferentially responds with IgA
antibody