The immune system consists of primary and secondary lymphoid organs. The primary organs, bone marrow and thymus, are responsible for the development and maturation of immune cells from hematopoietic stem cells. The secondary organs, including spleen, lymph nodes, and mucosa-associated lymphoid tissue, are where immune responses are initiated through interactions between lymphocytes and antigens. Hematopoietic stem cells in the bone marrow differentiate into mature blood cells, including lymphocytes, which then travel to secondary organs to carry out immune functions.

1. Cells and organs of Immune
system
Parmar Krupal

2. Classification of organ of immune
system
• Primary Lymphoid Organs—Where Immune
Cells develop
• Site for development and maturation of lymphocytes.
• i.e. the bone marrow and the thymus—regulate the
development of immune cells from immature
precursors.

3. • Secondary Lymphoid Organs—Where the
Immune response Is Initiated
• Sites for lymphocytes and antigen interaction.
• i.e. including the spleen, lymph nodes, and specialized
sites in the gut and other mucosal tissues, MALT
Diagram of a lymph
node, showing the
flow of lymph
through the lymph
sinuses.

5. • hematopoietic stem cell (HSC) : all functionally
specialized, mature blood cells (red blood cells,
granulocytes, macrophages, dendritic cells, and
lymphocytes) arise from a single cell type.
• The process by which HSCs differentiate into mature
blood cells is called hematopoiesis.

6. • Two primary lymphoid organs are responsible
for the development of stem cells into mature
immune cells:
• the bone marrow,
– where HSCs reside and give rise to all cell types; and
• thymus,
– where T cells complete their maturation

7. • Stem cells are defined by two capacities:
• (1) the ability to regenerate or “self-renew”
• (2) the ability to differentiate into all diverse cell types.
• Types of stem cell
» Embryogenic stem cell
» Adult stem cell
capacity to generate every specialized cell type in an
organism( in other words, they are pluripotent )
have the capacity to give rise to the diverse cell
types that specify a particular tissue. Multiple adult organs
harbor stem cells (“adult stem cells”) that can give rise to
mature tissue-specific cells.

8. •Self-renewing hematopoietic stem cells give rise to lymphoid and myeloid progenitors.
• Most immune cells mature in the bone marrow and then travel to peripheral organs via
the blood. Some, including mast cells and macrophages, undergo further
maturation outside the bone marrow. T cells develop to maturity in the thymus.

11. • HSC can become a common myeloid-
erythroid progenitor (CMP), which gives rise
to all red blood cells (the erythroid lineage),
granulocytes, monocytes, and macrophages
(the myeloid lineage), or
• Either it can become a common lymphoid
progenitor (CLP), which gives rise to B
lymphocytes, T lymphocytes, and NK cells.

13. • Regulation of Lineage Commitment during Hematopoiesis
• Multiple genes that specify lineage commitment have been identified.
Many of these are transcriptional regulators
Transcription factor Importance
GATA-2 required form the development of all hematopoietic lineages. in
its absence animals die during embryogenesis.
Bmi-1 It is required for the self-renewal capacity of
HSCs, and in its absence animals die within 2 months of birth
because of the failure to repopulate their red and white blood
cells.
Ikaros is required for lymphoid but not myeloid development; animals
survive in its absence but cannot mount a full immune response
(i.e., they are severely immunocompromised).
Notch1 regulates the choice between T and B lymphocyte lineages