ANS102 Lecture Notes - Lecture 15: Cfu-Gemm, Lymphopoiesis, Innate Immune System
The immune system is made up of specialised cell types, many of which can be seen
circulating in the blood. These cells are called white blood cells or leukocytes and they
originate from haematopoetic stem cells in the bone marrow. When blood is stained on a
slide using various dyes, leukocytes appear to be either granular (containing lots of small
granules) and are called granulocytes, or they appear agranular with a simple nucleus shape
and are called mononuclear cells. Granulocytes include neutrophils, eosinophils, basophils.
Mononuclear cells include monocytes and lymphocytes. The total number of these cells in
the blood is often measured clinically and reported as the WBC (white blood cell count). The
WBC can be diagnostic for some disorders such as infection and cancer of the blood
(leukaemia). Leukocytes can also be found in the peripheral tissues of the body where they
further differentiate into other cells (such as macrophages and dendritic cells which derive
from monocytes) or become activated (such as neutrophils). Lymphocytes are often found
in the lymphoid organs of the body. Collectively, the leukocytes in blood and lymphoid
organs make up the innate and adaptive immune defences of the body.
Myeloid and Lymphoid cells – haematopoetic stem cells given rise to two progenitor
types. The common lymphoid progenitor gives rise to the lymphocytes – small mononuclear
cells with little cytoplasm – which include Natural Killer (NK) cells, T cells and B cells. The
common myeloid progenitor gives rise ultimately to red blood cells, platelets, all of the
granulocytes and the monocyte. The monocyte then differentiates further in tissues to
become either a macrophage or a dendritic cell. The mast cell, a large granulocyte found in
tissues, is also derived from the myeloid progenitor, but its intermediary cell is not yet
known. All myeloid cells and NK cells are members of the innate immune system, whereas B
cells and T cells are uniquely part of the adaptive immune system. This is because only T
cells and B cells can make unique receptors on their surface from shuffled gene-segments,
and only T cells and B cells can make multiple copies (clones) of themselves. Every cell in the
immune system has a specific function and plays an important part in immune defence.
Primary and Secondary Lymphoid Organs – home for T and B lymphocytes – It is
not possible to understand T cell and B cell biology without first examining the immune
organs where they are made and function. Lymphoid organs are complex tissues containing
T cells and B cells in highly structured arrangement surrounded by non-immune
cells. Primary lymphoid organs are where B cells and T cells mature and make their unique
antigen receptors and remove any self-reactive clones – B cells mature in the bone
marrow and T cells in the thymus. Once mature, naïve T cells and B cells (naïve because
they haven’t yet seen foreign antigens) circulate between the blood and secondary
lymphoid organs which include the spleen, lymph nodes, tonsils and other tissue-specific
structures. The secondary lymphoid organs are where immune responses start.
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Document Summary
The immune system is made up of specialised cell types, many of which can be seen circulating in the blood. These cells are called white blood cells or leukocytes and they originate from haematopoetic stem cells in the bone marrow. When blood is stained on a slide using various dyes, leukocytes appear to be either granular (containing lots of small granules) and are called granulocytes, or they appear agranular with a simple nucleus shape and are called mononuclear cells. The total number of these cells in the blood is often measured clinically and reported as the wbc (white blood cell count). Wbc can be diagnostic for some disorders such as infection and cancer of the blood (leukaemia). Leukocytes can also be found in the peripheral tissues of the body where they further differentiate into other cells (such as macrophages and dendritic cells which derive from monocytes) or become activated (such as neutrophils).
