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Lymphoid Tissues and Organs:
- Leukocytes may be distributed in the body as:
1-Single cells in tissues and circulation.
2-Lymphoid accumulations (Peyer’s patches).
3-Aggregations within Lymphoid organs; Primary and
secondary lymphoid organs.
-Primary Lymphoid Organs: Thymus and Bone marrow.
-Secondary Lymphoid Organs: Spleen, Lymph nodes,
Tonsils, and MALT.
Primary and Secondary Lymphoid Organs:
Primary lymphoid organs:
1- Thymus:
-T Lymphocytes develop within this lymphoid organ.
-Function: The clonal selection of T lymphocytes.
(Lymphocyte educational Center)
-It increases in size during fetal and neonatal life.
-It is progressively inactivated (curved spirally) following
-Two important parts:
1-Thymic Cortex.
2-Thymic Medulla.
Primary lymphoid organs: The Thymus and its parts.
The thymus diagram :
Parts and location.
2-Bone marrow:
-B lymphocytes are “home schooled” within this organs.
Primary differentiation of B lymphocytes.
B lymphocytes begin to display IgM on their surfaces.
-The primary site for cytokines-Immune cell interactions.
-Bone marrow
removes the B cells
that show selfreactivity by
Secondary Lymphoid Organs:
-Secondary lymphoid organs function as filtration devices
removing foreign matter, dead cells, and microbial toxins
from the circulation.
-Blood vessels and lymphatic vessels richly supply these
-Specialized regions of the vasculature (endothelial venules)
facilitate movement of immune cells between blood and
the tissues of these organs.
-The leukocyte-rich nature stimulate cellular interaction.
Secondary Lymphoid Organs:
1-Lymph nodes:
-It acts as filters to purify lymph.
-Divided into the cortex and medulla.
-The superficial cortex
contains lymphocyte-rich
nodules (follicles)
(mainly B cells).
-The deep cortex is the
T-cell-rich area.
2-The Spleen:
-The largest lymphoid organ.
-Concentrates blood-borne antigens and microbes.
-Contains T cells, B cells, and Large numbers of plasma
cells (secreting immunoglobulins into the circulation).
-Divided into:
white pulp.
red pulp (also
3-Mucosa-associated lymphoid tissues(MALT):
-Other sites for immune-cells interaction.
-Tonsils in the nasopharynx.
-Peyer’s patches in the sub-mucosal surfaces of small
The Lymphatic circulatory system:
-Leukocytes and their products use two circulatory
1-Cardiovascular system.
2-Lymphatic circulatory system. (Textbook : Page 85-86).
Lymphocyte Development:
-Stem cells of bone marrow (prothymocytes; negative for
CD4,CD8, and TCR) migrate via the circulation to the
thymic cortex.
-The newly arrived thymocytes acquire CD4,8, and TCR
(Positive cells)
-Cortical thymocytes are selected by their interaction to
cortical epithelial cells (positive selection).
-Medullary thymocytes are selected (negative selection).
-Mature T cells are released into the circulation.
Maturation of T lymphocytes in Thymus:
Lymphocyte Activation:
Lymphocyte activation occurs according to following
1-Antigen endocytosis and processing.
2-Antigen presentation.
-Extracellular microbes and toxins are engulfed by
endocytosis.(Endocytic pathway).
-Intracellular microbes (viruses) are processed by cytolytic
Endocytosis of extracellular microbes:
2-Receptor-mediated endocytosis.
Antigen presentation by MHC Class II:
-Endocytic vesicles (phagosomes; derived from phagocytosis)
fuse with lysosomes.
-Phagolysosome interacts with endoplasmic reticulum
-The digested
peptides carried
by MHC II to cell
Antigen presentation by MHC Class I:
-Cytoplasmic protein-ubiquitin reaction.
-Proteasome action on the complex.
-Fusion of processed peptide with E.R vesicle which carry
MHC Class I to the cell surface.
Activation of T Lymphocytes:
-MHC II-epitope-Complex interaction with TCR.
-B7(CD80/86) Co-stimulatory interaction with CD28.
-IL-2 production from activated T cell.
-Overexpression of IL-2R on T cell surface.
-In the presence of IL-12, native CD4 lymphocyte is
differentiated as a CD4 Th1.
Activation of CD8 (Tc) cell by effector Th1:
Activation of B Lymphocytes by effector Th2:
-MHC II-epitope-Complex interaction with TCR.
-CD40-CD40 Ligand interaction.
-Antibody-epitope interaction.
Lymphocyte Effector Functions:
Cell-mediated immunity:
1-Role of CD4 T cells in Delayed (- type) hypersensitivity:
2-Role of CD8 T cell: T cell Cytotoxicity :
A-Target cell recognition.
B-Target cell destruction:
-Perforins and Granzymes effect.
3-Role of NK cells in cellular immunity:
Humoral immunity:
1-Agglutination of invaders.
2-Neutralization (virus, toxins).
4-Complement activation.
5-ADCC: NK cell and Eosinophils.
6-Immediate hypersensitivity.
Humoral immunity and effector lymphocyte function:
The ADCC Mechanism: