Immunology: Organs of the Immune System

• Immunologie Fondamentale for first year medical students, academic year 2024-2025, Professor Abdelhamid ZRARA, Mohammed V Military Hospital - Rabat

The Organs of the Immune System

• The immune system (IS) safeguards the body and fights against threats, using effectors

Plan

• Introduction
• Central Lymphoid Organs
• Peripheral Lymphoid Organs
• Conclusion

Introduction: Two Types of Organs

• There are primary or central lymphoid organs

• These include the bone marrow (MO)

• Also includes the thymus

• The function is the production and maturation of immune system cells

• There are secondary or peripheral lymphoid organs

• These include lymph nodes

• Includes the rate

• Includes also mucosa-associated lymphoid tissue (MALT)

• Activation, proliferation, and differentiation of lymphocytes into effector cells happens here

• The cells here encounter antigens

• The organs and tissues of the immune system are interconnected by lymphatic and blood circulations

Central Lymphoid Organs

• The central lymphoid organs are positioned centrally within the body to give

Moelle Osseuse

• The bone marrow (MO) functions as a multicentric factory
• Consists of cavities in flat bones like the sternum, ribs, vertebrae, and sacrum.
• Found in the epiphyses of long bones like the femur and humerus
• Composed of a network of vascularized fibrils with blood sinuses
• Contains fat cells (yellow marrow) and hematopoietic tissue (red marrow)
• The hematopoietic tissue includes erythroid and platelet tissue, myeloid tissue, and diffuse lymphoid tissue

Moelle Osseuse - Hematopoiesis

• Bone marrow (MO) hosts hematopoiesis

• The hematopoietic stem cell (CSH) gives rise to immune system cells

• CSH as 2 fundamental properties

• It programs its products on demand

• Capable of self-renewal

• Cytokines stimulate the maturation and differentiation of the following:

• Lymphoid stem cells

• Myeloid stem cells

• Lymphocytes:

• LT leaves the MO

• LB remains in the MO and leaves only when mature

Thymus

• The thymus is located, in the upper, anterior mediastinum, between the sternum and the great vessels
• Its structured in a bilobed fashion
• It's encapsulated, the capsule emits connective tissue, dividing into lobules
• Composed of two zones
• Cortical Zone
• Medullar Zone

Thymus - Cortical Zone

• The cortical zone is at the periphery
• It is rich in immature T cells (thymocytes)

Thymus - Medullar Zone

• The medullar zone is in the center
• Poor in thymocytes
• The architecture contains macrophages, interdigitated dendritic cells(DC)
• The region contains epithelial cells forming Hassall's corpuscles.

Thymus - Maturation

• The thymus receives T-cell precursors that migrate from the bone marrow (MO)
• The thymocytes are subjected to the maturation process inside the thymus
• This process involves T-cell receptor (TCR) acquisition, double positive and negative selection, and multiplication within a microenvironment containing cytokines and hormones
• 95% of T cells undergo apoptosis, dying in place
• 5% of T cells reach maturity, and leave for the periphery
• Those with appropriate, rapidly responsive T-cell receptors can be selected
• Negative selection in the thymus eliminates T cells that react strongly to self-antigens to present autoimmune conditions

Peripheral Lymphoid Organs

• Peripheral Lymphoid Organs the place where there is immunologic risk

Organization of the Lymphatic System

• Blood circulation allows plasma to filter across capillaries into tissues, forming interstitial fluid.
• Part of it returns into blood circulation where the remainder is called lymph
• Lymph is absorbed by lymphatic capillaries and travels inside lymphatic vessels
• The lymphatic vessels carry lymph until the thoracic canal which flows into the left subclavian vein.
• Lymph vessels deliver antigens to peripheral lymphoid organs
• Lymph is how lymphocytes travel through connective tissues and peripheral lymphoid organs

Organization of Lymphoid Tissue

• Part of the lymphoid tissue is organized in lymphoid follicles
• Lymphatic capillaries surround the DC folliculaires and LB
• Lymphoid follicle states of activity:
• At rest, it is "primary follicle"
• It becomes “secondary follicle” after being stimulated
• LB forms a ring around the germinal center
• The germinal center is composed of LB and elevated with DC and macrophage rates
• Ganglions and the rate are the most organized lymph nodes

Ganglions

• Ganglions are a thousand lymph nodes spread in points through the organism
• Superficial locations include : cervical, axillary, and inguinal
• Profound locations include : mediastinal and abdominal
• Globular/ reniform with a depression: known as the hile
• Lymph ganglions run along vascular system and are the loci of immunologic activity

Ganglions structure

• The sinus capsulaire surrounds the GG and divides it in septa.
• Ganglion tissue can be divided into three portions:
• Cortical
• Has lots of LB, macrophages, DC folliculaires
• Para-cortical
• Contains LT and DC interdigitées
• Medullar
• Lacks lymphocytes, but contains some plasmocytes and/or macrophgaes

Lymphocyte entry

• Afferent Lymph Vessels carry cells into the Ganglions
• At the cortical level, antigens that have undergone an immune response will disperse along the vessels
• At the para-cortical level, capillary veins differentiate into High Endothelial Venules (HEV)
• HEV has a key role in the recirculation of lymphocytes from the blood

Rate

• The rate is located in the left hypochondrium, between the stomach and diaphragm
• It's ovoid
• Border is crenelated
• Hile allows passage of the splenic vein and artery
• Functions as the largest lymphatic organ
• It is hemo-lymphatic
• It doesn't affect lymph nodes, but is highly vascular
• Allows filtration of antigens from the blood

Rate - Structures

• The capsule of the rate emits branches that form lobes
• Has parenchym composed of 3 zones:
• Red pulpe
• Composed of sinus with GR and macrophages
• White Plupe
• Composed of arteriole that form PALS with LT/DC interdigitées
• Marginal Zone
• Forms the boundary with LB and follicules, macrophages and DC follicules

Rate - Lymphocyte interaction

• Microbic antigens and lymphocytes are conveyed by the splenic artery to PALS
• The cells respond in contact with : -DC interdigitées where they respond with LT, or -DC folliculaires where they respond with LB in the marginal zone to generate plasmocytes

MALT

• There is Mucosal Associated Lymphoid Tissue
• Includes GALT = Gut Associated Lymphoid Tissues from the digestive tract, namely, the appendix
• The plaques of peyer that line the grêle.
• BALT = Bronchial Associated Lymphoid Tissues in the respiratory tract that affects végétations adenoïde
• DALT= (Duct Associated Lymphoid Tissues in the glandular system involved with activating proliferation

Conclusion

• Central lymphoid organs are where lymphocytes mature
• The primary Lymphoid tissue gives rise to lymphocytes
• Peripheral Lymphoid organs are the location that responses are triggered
• Anomalies in the organs give rise to immunity deficiences, affecting the production and/ or différentiation of lymphocytes