MCD – Immunology 2 - Immune Cells and Organs
Anil Chopra
1. Explain the use of CD (cluster of differentiation) markers.
Both B and T lymphocytes look the same under a microscope however they can be
differentiated using CD (clusters of differentiation) markers. This is a nomenclature
used for cell surface proteins on cells of the hematopoietic system – cells of the blood
(produced by stem cells).
2. Compare and contrast the phenotypic characteristics of T and B lymphocytes and
T lymphocyte subsets
T-lymphocytes recognise antigens that are
presented on the surfaces of other cells by the MHC
major histocompatibility complex.
B- lymphocytes recognise antigens that are free in
the blood (or any fluids).
3. Name the primary and secondary lymphoid
organs
4. Briefly differentiate between their functions.
5. Draw simple diagrams to illustrate the structure
of the thymus, lymph node, spleen, Peyer’s
patch and indicate the changes that occur after
stimulation by antigen.
 The thymus is a primary lymphoid organ. It
produces T-cells.
 Thymic output decreases with age which is why pneumonia and influenza is such
a big killer of the elderly.
Primary Immunodefiency
Syndromes include:
- Di George Syndrome –
where there is a defect in
the development of the foetus. Patients are given
a thymus transplant.
-
Severe Combined Immune Deficiency (SCID) – this is where there is a
defect in lymphoid stem cells. As a result, no T cells develop and in some
cases no B cells develop. Patients are given a bone marrow transplant.
Secondary Immunodeficiency
syndromes occur when damage
occurs to the T-cells themselves
by irradiation,
immunosuppressive drugs, some
environmental agents and viruses.

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The bone marrow is a primary lymphoid organ and
the site of production of B-lymphocytes.
This occurs in the centre of the bone marrow with the cells
becoming more and more mature as they
get closer to the centre of the bone.
In primary immunodeficiency, there may
be lesions causing stem cells not to develop into Bcells and reach the centre of the bones.
In order to make sure that the lymphocytes come into contact with the antigens, the
secondary lymphoid organs have special vascular adaptations to recruit lymphocytes
from the blood. These organs include the spleen, lymph nodes and MALT (mucosal
associated lymph tissues).
About 3 litres of fluid passes through the lymph system every day. This fluid moves
incredibly slowly and as a result of skeletal muscle movement.
primary
follicle
germinal
center
secondary
follicle
In a normal immune response it is the T- helper cells (CD4) that co-ordinate other
cells in the reactions to the antigen. It:
-
Releases cytokines (IL-2 – interleukin 2, IFN- - interferon , and TNF – tumour
necrosis factor) which attract T-killer cells (CD8) and activates them to dispose
of the antigen. (cells that do this are known as T-helper or Th-1 cells)
-
Other cytokines (IL-4, IL-5 and IL-6) also cause the clonal expansion of Blymphocytes (cells that do this are known as Th-2 cells).
6. Give examples of professional antigen presenting cells (APCs) and state their
locations.
APCs are antigen presenting cells that present peptides to T-cells in order to initiate
the immune response. They include:
7. Outline the recirculation of lymphocytes.
Many lymphocytes die after the antigen is cleared.