The lymphoid organs are classified
into two categories:
• 1 Primary (central) lymphoid organs are responsible
for the development and maturation of lymphocytes
into mature, immunocompetent cells.
• 2 Secondary (peripheral) lymphoid organs are
responsible for the proper environment in which
immunocompetent cells can react with each other, as
well as with antigens and other cells, to mount an
immunological challenge against invading antigens or
pathogens.
Central organs
• 1.Red bone marrow:
• Is located in cellulae of spongy bone matrix forming inner
part of flat and spongy bones and epiphysis of tubular
bones.
• In childhood(till 12-18)RBM fills also central cavity of
diaphysis of tubular bones(later is removed by yellow bone
marrow).
• RBM has semifluid consistence. Common weight in adult
is 3,0-3,5 kg.
Lymphoid tissue
Function: keeping the own antigen environment, eliminating the
foreign substances/antigens
Lymphoid tissue of lymphoid organs:
- Primary (red bone marrow, thymus): development of RBC, WBC,
Platelets, Lymphocytes
- Secondary (lymph node, spleen, tonsils, Payers patches, appendix):
mature Lymphocytes can be found here
Stromal tissue: reticular connective tissue (reticular fibers, reticular
сells), reticular-epithelial cells (thymus)
Main cell types: lymphocytes, macrophages, dendritic cells,
endothelial cells
Red
bone
marrow
1.BLOOD CELLS: almost all cells of haemopoesis (except terminal
stages of maturing of lymphocytes) are located in RBM.
Haemopoetic cells are distributed by islets. They are mainly cells of
V classes:
A)Erythropoetic islet:
1)IV class: proerythroblasts;
2)Mainly V class:
• basophillic erythroblasts,
• polychromatophillic erythrobl,
• oxyphillic erythroblasts,
• reticulocytes and erythrocytes.
B)Granulocytopoetic islets: neutrophillic, basophillic,
eosinophillic.
1)IV class: myeloblasts;
2)Mainly V class:
• promyelocytes,
• myelocytes(N,B,E),
• metamyelocytes(N,B,E),
• band granulocytes(N,B,E),
• less of segmented cells.
C)Trombocytopoetic islet:
1)IV class: meakaryoblasts;
2)V class: promagekaryocytes and megakaryocytes.
D)Cells are similar to small lymphocytes:
1)Low-differentiated cells of I-III classes, are located
between islets.
2)More mature cells of monocyte’s and B-lymphocyte’s
rows.
• In humans, the fetal liver, prenatal and postnatal bone
marrow, and thymus constitute the primary lymphoid organs.
• The lymph nodes, spleen, and mucosa-associated lymphoid
tissues (as well as the postnatal bone marrow) constitute the
secondary lymphoid organs.
Thymus
• Is located behind the sternum. Weight of thymus is got its
maximum in 14-15 years-35-40 gr., after 20 years old it is
gradually decreased.
Following processes occur in thymus:
1)Finishing of maturing of T-lymphocytes and their further
proliferation.
2)Elimination (killing) of main part of T-lymphocytes which
are oriented against organism’s own antigens.
STRUCTURE
A) From the surface thymus is covered by capsule (made up of
DRCT). It gives numerous septa of connective tissue dividing thymus
onto lobules.
B) In each of lobule there are cortex(peripheral and dark) and
medulla(middle and light).
• Cortex and medulla consists of 4 components of:
1.Maturing or mature T-cells.
2.Epithelial stroma.
3.Macrophages.
4.Special blood vessels, lymphatic vessels.
THYMUS
The thymus is a primary
lymphoid organ that is the
site of maturation of T
lymphocytes.
• The thymus, situated in the superior mediastinum
and extending over the great vessels of the heart, is a
small encapsulated organ composed of two lobes.
• Each lobe arises separately in the third (and possibly
fourth) pharyngeal pouches of the embryo.
• The T-lymphocytes that enter the thymus to become
instructed to achieve immunological competence
arise from mesoderm.
Cortex of thymus
1.Lymphoid cells:
A) In subcapsullaris region of the cortex of lobules there are Tlymphoblasts (IV class). They are much larger and lighter than mature
cells, have numerous divisions.
Main part of cortex is present by maturing T-lymphocytes (V class)
B) Functions of these cells:
1) Rearrangement of genes-formation of complete gene coding TCR
with its immunospecifity.
2) 2 tours of selection of cells depending on nature of their TCR.
• During these processes T-cells are moving from cortex to medulla.
• Duration of maturing of T-cells is 20 days.
Medulla
1.Blood cells: T-lymphocytes, less than in cortex.
a) There are less number of maturing T-lymphocytes
transferred from cortex
b) Main part of cells in medulla are recirculating T-cells
(several times entering the thymus from blood and again
returning in to blood for providing their functions). It is
possible due to absence of barrier in medulla