The Immune system

The Immune system
The immune system consists of specialized cells and organs that protect an
organism from outside biological effects and from inside changes.
If the immune system is functioning properly, it protects the body against
microbial infections, destroying cancer cells and foreign substances.
If the immune system weakens, it allows pathogens to grow and flourish.
Organs of the immune system
Central (primary) lymphoid organs
These are the sites of maturation of the lymphocytes
1-Bone Marrow:
The source of erythrocytes, platelets, white blood cells.
2-Thymus:
is a specialized primary lymphoid organ of the immune system. Within the
thymus, T cells mature.
It consists of two parts:
Medulla:
The site of the thymic corpuscles
Cortex:
Rich in lymphocytes.
Peripheral (secondary) lymphoid organs:
These are the site interaction between the lymphocytes and the antigen.
1-Lymph nodes:
The lymphocytes leave the circulation and enter the node then migrate towards
the medulla then return to the circulation via Afferent lymphatic channels that
drain into the thoracic duct
2-Spleen:
Synthesis of immunoglobulins , phagocytosis of blood particles and antigens.

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,2. Mucosa-associated lymph01d tissue (MALI);
Scattered, non-encapsulated Iymphoid tissues
of mucosal surfaces like
gut-associated lymphoid tissue (GALT)
bronchus-associated lymphoid tissue (BALT)
3 Tonsils:
Exposed to both air-borne and alimentary
antigens
Cells Involved in the Immune Response
All the cells of the blood, including the cells of the immune system arise from
single pluripotent hematopoietic stem cells in the bone marrow.
Stem cells can give rise to all the different types of blood cells
i.e. they divide to produce two types of stem cells.
1-A common lymphoid progenitor cell that give rise to:
a. T lymphocytes: T helper cell (Th) and T cytotoxic (Tc)
b. B lymphocytes which give plasma cells that secret antibodies.
C. Natural killer (NK) cells.
2-A common myeloid progenitor that give rise to:
a-Leukocytes:
Neutrophils .Eosinophils Basophils Mast cells Dendritic cells
Monocytes (monocytes in blood while called macrophages in tissue)
b-Erythrocytes
C-Platelets




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,Normal differential WBCs counts:
Total WBC 4,300-10,800/mm3
Total Neutrophils 2500-8000 (55-70%) –
Bands 0-700 (0-5%) –
Segmented 1800-7000 (54-65%)
Lymphocytes 1000-4000 (25-40%)
T cells 600-2400
B cells 50-250
Monocytes 100-700 (2-8%)
Eosinophils 50-500 (1-4%)
Basophils 25-100 (0-1%)
Lymphocytes
There are three types of lymphocytes:
1-Tlymphocytes: mediators of cell-mediated immunity.
2-B lymphocytes: mediators of humoral immunity.
3-Natural killer cells: cells of innate immunity.



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, 1-T Iymphocytes:
Characters:
Represent 45-70%o of total blood lymphocytes.
Derived from bone marrow.
Developed in the thymus.
Function:
Cell-mediated immunity (discussed later in acquired immune response)
Types:
According to protein known as CD (Cluster of Differentiation) T-cells have either
CD4or CD8 proteins on their surface,
cluster of differentiation 4 on T helper cells
cluster of differentiation 8 on T cytotoxic cells
T helper (CD4) :
Called helper because they Help other cells through secretion of helper factors
i.e. cytokines
Help B lymphocytes activation to produce antibodies.
Help macrophage activation to destroy ingested microbes.
Subsets:
Th1 & Th2 according to cytokine profile.
Th1 function:
Secrete IFN-y which activates phagocytes to kill microbe.
Activate B lymphocytes to proliferate to plasma cells (that secrete antibodies)
and to memory cells.
Activate macrophages to destroy the ingested antigens.
Th2 function:
Secrete IL-4 & IL-5, which stimulate IgE and eosinophil/ mast cell degranulation
in case of allergy and helminthes reaction.
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