HYPERSENSITIVITY TYPE IV

HypersensitivityDEFINITION

“ excessive and undesirable reaction

produced by normal immune system.”

Hypersensitivity Type IV General conceptDelayed type hypersensitivity ( takes 2 or 3 days to develop).

No antigen antibody mediated response.

Cell mediated response.

Cells involved in Hypersensitivity Type IVT Lymphocytes : are further divided into two types.

Helper T Cells (CD4)Cytotoxic T Cells (CD8)

Antigen Recognition T cell fail to recognize antigen in the

absence of Antigen presentation.T cell receptor is restricted to

recognize antigenic peptide only when bound to the appropriate molecule major histo compatibility complex (MHC).

This is called MHC restriction.

Two Categories of Antigen Are processed and presented to T cell by two different mechanism.

Exogenous AntigenEndogenous Antigen

Exogenous Antigen

Present MH class II.Are taken by the Antigen presenting cells

(Dendritic cells ; Macrophages).

Antigen Presenting Cells:

Engulf the antigen by endocytosis.Endosome fuses with the Lysosome.So antigen is degraded into fragments.Display on cell surface by combining with

MHCIIHere they may be presented to CD4 helper T

cells.CD4 helper T cells then become activated.

Function of Activated CD4 (Helper T cells)Responsible for regulatory

functions.Produce interleukinInterleukin 2 activates CD4 and CD8

also called as T cell growth factor.Interleukin 4 and 5 are activator of

B cells.Gamma Interferon activates

macrophages; NK cells; neutrophils.

Function of Activated CD4 (Helper T cells)

Endogenous AntigenPresent MHC1 class.Antigen present within the cell e.g, viral

protein in any infected cell are;Degraded into peptide fragment.Peptide fragment form a complex with

MHC I.Displayed at the cell surface.Here they may recognize by CD8

( Cytotoxic Cell).

Function of Activated CD8 cell

Perforins: formation of channels through the membrane.

Pore formation.Cell content is lost.Cell dies.

CD8 ( Cytotoxic T Cells)Granzymes: Proteases Degrade protein in cell membrane loss of cell content

Cell dies

Activation of CD8 (Cytotoxic T Cells)

Function of Activated CD8 (Cytoxic T Cells)