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Cells and Organs of the
Immune System
Cells – Leukocytes
a. Lymphocytes – specialized for adaptive immunity
b. Granulocytes/Agranulocytes – func in accessory roles in
adaptive immunity
Immune Organs – 2 major groups
a. 1° lymphoid organs – where lymphocytes dev. and mature
b. 2° lymphoid organs – where lympho’s interact w/ Ag
• Begins with hematopoietic stem cells (HSC)
– Few in # in bone marrow; difficult to culture
– Pluripotent; able to produce RBC’s, WBC’s, megakaryocytes
• HSC differentiates to become:
either a) Myeloid progenitor cell
or b) Lymphoid progenitor cell
Myeloid RBC’s and WBC’s and dendritic cells
Lymphoid B and T cells and dendritic cells
Fig 2-1 Kuby, 5e
Hematopoietic influences
• Stromal cells influence diff of HSC by providing hematopoieticinducing microenvironment (HIM) with:
-colony stimulating factor (CSF)
-erythropoietin (EPO)
• Cytokine influences -> stimulated by activated MØ and T cells
• Genetic regulation – see Table 2-1.
• Hematopoietic homeostasis – req’d to maintain certain [c] of cells
balance between cells removed by cell death and those produced
-for WBC’s: a human must produce ~3.7 x 1011/day !
*Each WBC has a specific life span + dies by apoptosis; Neutrophils live
only a few days
Apoptosis – programmed cell death
Process thru which cells die + later phago’d
• decrease in cytoplasmic volume; apoptotic bodies
• Clumping/break up of DNA
• Phago’d by MØ  blocks release of cyto contents
no local inflam response
It is influenced by gene expression
• Chief among the genes is bcl2 - inhibits cell death
(Activ. lympho’s express lower levels of bcl2 and are more
susceptible to apoptosis)
-If activ continues, the apoptic signal is blocked
-If activ does not, lympho’s begin to die by apoptosis
Fig 2-5
Hematopoietic stem cells
can be enriched