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Structure & Function of Immune System

A. Cells of the immune system



B.  Important Points

  1. Lymphoreticular system :  lymphoid and reticuloendothelial components
  2. Lymphoid cells : Lymphocytes and plasma cells- specific immune response
  3. RE System; Phagocytic cells-scavengers-eliminate effete cells and foreign particles
  4. Non specific immunity –removing   microorganisms
  5. Humoral immunity: Antibodies mediated
  6. Antibodies produced by plasma cells
  7. Cellular immunity : Sensitized lymphocytes
  8. Cells –develop through separate channels remain independent may also interact 

C.  Lymphoid system

  1. Lymphoid cells –lymphocytes and plasma cells
  2. Lymphoid organs
  3. Central and Peripheral
  4. Central (Primary)-thymus and bone marrow
  5. Peripheral – Spleen, Lymph nodes and MALT(Mucosa associated lymphoid tissue) Lymphoid
  6. tissue – gut, liver, lungs and bone marrow 

D. Thymus

  1. First organ to become predominantly lymphoid
  2. Primary function-production of thymic lymphocytes
  3. Lymphocytes acquire new surface Ag.(Thy Antigens)
  4. Lymphocytes are called Thymus (T)dependent lymphocytes or T cells
  5. Lymphocyte proliferation not dependent on any antigenic stimuli

E.  T Lymphocyte

  1. Thymus confers immunological competence on the lymphocytes- they are educated.
  2. Seeded selectively into certain areas
    1. White pulp of spleen
    2. Central arterioles
    3. Paracortical areas of lymph nodes 

F.  These are the Thymus dependent regions

  1. T cell precursors –migrate –from yolk sac, fetal liver and bone marrow- to thymus
  2. Earliest one –CD 7 pro T Cells
  3. They entre thymus –acquire CD2
  4. Synthesize CD3 in cytoplasm-become pre T cells
  5. T cell receptor (TCR)is also synthesized 


  1. Glycoprotein chains-
  2. Expressed on T cell surface
  3. Act as Antigen recognition unit in association with CD3
  4. Glycoprotein chains- αβ and λδ
  5. Pre T cells differentiate into two expressing either of these chains
  6. Large majority carry αβ TCR
  7. Four separately coded regions
    1. Variable-V
    2. Diversity-D
    3. Joining-J
    4. Constant –C
      Hence belong to Immunoglobuluin gene super family
  8. Wide repertoire of Ag Specificity by re assortment of these regions
  9. Destruction of Immature T cells carrying TCR in thymus-Contact with self antigens
  10. T cells develop MHC restriction so
    a.  CD8 cells respond to Ag presented along HLA class I
    b.  CD 4 cells to Class II
  11. Immature T cells in thymus-CD7,2,3,1,4,and 8 and TCR
  12. On maturity lose CD1 and differentiate into CD4 and CD8
  13. CD4-helper inducer cells,
    a.  Induce B cell differentiation,
    b.  Stimulate proliferation of CD8 cytotoxic cells
    c.   Regulate certain stages of erythropoiesis
  14. CD8 cells
    1. Suppressor/cytotoxic cells
    2. Inhibit B cell Ab synthesis
    3. Act as cytotoxic effector cells 

H. B Cell

  1. Pro B cells develop in Fetal liver and then in the bone marrow through out life
  2. Rearrangement of Ig takes place in becoming Pre B cells-secrete Cytoplasmic IgM
  3. Immature B cells –IgM expressed on the cell surface
  4. Migrate to periphery – undergo Ig switching
  5. Other than IgM the cells also express other Ig classes
  6. Reassortment
  7. Capacity developed to produce Ig molecules
  8. Allelic exclusion
  9. B cell programmed to form only one class of Ig with either kappa or lambda chain
  10. Contact with Ag –mature B cell undergoes proliferation
  11. Majority of B cells transformed into plasma cells  

I.   Plasma cells

  1. Antibody secreting cells(Ig producing factory)
  2. Specific –Ab of single specificity, single Ig class, single light chain type only
  3. Exception-Primary immune repsonse-when initially IgM then IgG
  4. Other producing Antibody are Lymphocytes and lymphoblasts 

J.   Lymphocytes

  1. Found in peripheral blood, lymph, lymphoid organs and many other tissues
  2. Peripheral blood -20-25% of leucocyte population
  3. Human body -10 12 lymphocytes approx 10 9 renewed daily
  4. 1% of them found in blood
  5. Short lived –effector cells in immune response
  6. Long lived –storehouse for Ig memory ,mainly thymus derived
  7. Lymphopoiesis - central lymphoid organs
  8. Then circulation and finally peripheral lymphoid organs and tissues
  9. Mixing together lymphocyte recirculation
  10. Recirculating lymphocytes –T cells
  11. Lymphocyte educated by central lymphoid organ becomes ICC
  12. Mature B and T cells before encountering Ag are called Naïve cells
  13. Antigen recognition mechanism on their surface- each cell only one Ag.
  14. Stimulated T cells produce CMI
  15. Stimulated B cells –transform into plasma cells –synthesize Ig
    a.  T cells (CD 3)
    i.    Helper T cells (CD 4)
    ii.    Cytotoxic T cells (CD8)
    b. cells
  16. Null cells: 5% lymphocytes, no T cell or B cell marker
    a.  Killer Cell: subpopulation of null cells, receptor for Fc part of IgG, responsible for Antibody dependant cell cytotoxicity (ADCC)
    b.  Natural Killer Cells: subpopulation of null cells, also known as large granular lyphocytes (LGL), involved in allograft, tumor rejection, killing of virus transformed cells by production of perforins, CD16 and CD56 are the CD markers present on their surface, Activated by IL2 and IL12.
  17. Plasma cells: derived from activated B cells, secrete antibodies
  18. Neutrophils: 60% of circulating leukocytes, phagocytosis of foreign, dead cells, inflammation
  19. Eosinophils: 3-5% of circulating leukocytes, numbers raised in allergic and parasitic conditions
  20. Basophils: less than 1% of circulating leukocytes, tissue counterparts are called as mast cells. Contain mediators of type I hypersensitivity reaction. 
  21. Monocytes: 4-6% of circulating leukocytes.




Antigen recognition receptor

T cell receptor

M Igs

Surface antigen



Receptor for Fc piece of Ig



Receptor for C3 component of compliment



EAC rosette (C3 receptor, CR2, EBV receptor)



E rosette (CD2, measles receptor)



Thymus specific antigen



Blast transformation on treatment with

Phytohemagglutinin, Conconavalin A

EB virus, Endotoxin


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