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Natural Cells: (Nk Cells)


  1. Make up â 10-15% of peripheral blood lymphocytes: no T cell receptor/immuno globulin
  2. Also called large Granular lymphocytes - azurophilic granules Null cell, Non T Non B
  3. Part of natural immunity. 1st line of defense
  4. CD 16+ve     CD3-ve
  5. CD56 +ve
  6. Share some T cell markers (e.g. CD 2)
  7. CD16: Fc receptor for 19G -+ mediates in ADCC by ability to lyse 19G coated cells
    1. Two types of NK cell receptors
      1. Activating receptors: Activate NK cell killing by recognizing ill defined molecules on target cells
      2. Inhibiting receptors: Inhibits lytic pathway by identifying self class I MHC molecules (Normal all nucleated normal cells express self class I molecules which undergo change after infection/neoplastic change
        1. Secrete CKs like T NF, IFN -, GM-CSF
        2. Imp. for induction & regulation of immune response
        3. Major function.: To bind peptide fragment~ of foreign proteins for presentation to appropriate antigen specific T cells.
        4. Several genes code for HLA, most important ones are on chromosome 6 

MHC/HLA complex

(Because MHC encoded Ag were initially detected on white cells)


s I antigens:
Expressed on all nucleated cells & platelets.

-encoded by loci: HLA-A, HLA-B, HLA-C Q


MHC-I molecule

  1. Consists of a chain linked to β2 microglobulin (latter not encoded within MHC )
  2. Binds to peptides that are derived from proteins, Synthesized within the cell (e.g. Viral Ag)
  3. CD 8 molecules (co-receptor) binds to a3 domain Q
  4. Present antigens to CD 8 cells 

Class II Antigen:

  1. Encoded for in region called HLA-D (3 sub-regions: HLA-DP, HLA-DQ, HLA-DR) Q
  2. Hetero-dimer: made up of α & chains
  3. Class II molecules present exogenous antigens to CD4 cells.
  4. Present on Ag presenting cells only (macrophages, dendritic cells, monocytes)
  5. Class II (molecule) + Antigen - internalized - processed – MHC peptide complex - cell surface recognized by CD4 cell Q 

HLA and disease Association

Disease HLA Allele Relative Risk
Ankylosing spondylitis B27 90
Postgonococcal arthritis B27 14
Acute anterior uveitis B27 14
Rheumatoid arthritis DR4 4
Chronic active hepatitis DR3 13
Primary Sjögren syndrome DR3.. 9
Type - 1 diabetes DR3 5
  DR4 6
  DR3/DR4 20



Prototype Disorder

Immune Mechanisms

Pathologic Lesions

Immediate (type I) hypersensitivity

Anaphylaxis, allergies, bronchial asthma (atopic forms)

Production of IgE antibody immediate release of vasoactive amines and other mediators from mast cells; recruitment of inflammatory cells (late-phase reaction)

Vascular dilation, edema, smooth muscle contraction, mucus production, inflammation

Antibody-mediated (type II) hypersensitivity

Autoimmune hemolytic anemia; Goodpasture syndrome

Production of IgG, IgM binds to antigen on target cell or tissue phagocytosis or lysis of target cell by activated complement or Fc receptors; recruitment of leukocytes

Phagocytosis and lysis of cells; inflammation; in some diseases, functional derangements without cell or tissue injury

Immune complex-mediated (type III) hypersensitivity

Systemic lupus erythematosus; some forms of glomerulonephritis; serum sickness; Arthus reaction

Deposition of antigen-antibody complexes complement activation recruitment of leukocytes by complement products and Fc receptors release of enzymes and other toxic molecules

Inflammation, necrotizing vasculitis (fibrinoid necrosis)

T-cell-mediated (type IV) hypersensitivity

Contact dermatitis; multiple sclerosis; type I diabetes; transplant rejection; tuberculosis

Activated T lymphocytes (i) release of cytokines and macrophage activation; (ii) T-cell-mediated cytotoxicity

Perivascular cellular infiltrates, edema, cell destruction, granuloma formation

  1. B-cells are able to make a specific antibody against a specific antigen.
  2. It is due to presence of specific receptor on B-cells - B-cell receptor (BCR), that is usually an immunoglobulin Ig M or IgD.
  3. An antigen interacts with B-cell that shows best fit by virtue of its BCR.
  4. The antigen binds to this receptor, and the B-cell is stimulated to divide and form a clone (clonal selection).
  5. This clone of cells will become plasma cells that will secretes antibody of a particular specificity and same class.
  6. Although, B-cells are the major source of antibodies (after their conversion into plasma cells), Helper - T cells are also important.
  7. Helper T-cells activate B-cells by secreting cytokines (lymphokines).
  8. In antibody formation T-cells are involved earlier than B-cells.

Plasma cells synthesize an immunoglobulin of same specificity as that carried by the B-cell precursors
Also know

Class switching
Isotype or class switching is a biological process that changes an antibody from one class to another. For example from IgM (In primary response) to IgG (In secondary response)

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