Hypersensitivity Type IV is a delayed type hypersensitivity that involves cell-mediated responses without antigen-antibody interactions. T lymphocytes recognize antigens that have been processed and presented by antigen-presenting cells via MHC molecules. Activated CD4+ T cells secrete cytokines that regulate the immune response, while activated CD8+ T cells kill cells displaying endogenous antigens on their MHC Class I molecules through the release of perforins and granzymes. This leads to loss of cell content and cell death.

2. DEFINITION
“ excessive and undesirable reaction produced
by normal immune system.”

3. Hypersensitivity Type IV
General concept
Delayed type hypersensitivity (
takes 2 or 3 days to develop).
No antigen antibody mediated
response.
Cell mediated response.

4. Cells involved in Hypersensitivity
Type IV
T Lymphocytes : are further divided into
two types.
Helper T Cells (CD4)
Cytotoxic T Cells (CD8)

5. 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.

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

7. Exogenous Antigen
 Present MH class II.
 Are taken by the Antigen presenting cells (Dendritic
cells ; Macrophages).

8. 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 MHCII
 Here they may be presented to CD4 helper T cells.
 CD4 helper T cells then become activated.

9. Function of Activated CD4 (Helper
T cells)
 Responsible for regulatory functions.
 Produce interleukin
 Interleukin 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.

10. Function of Activated CD4 (Helper
T cells)

11. Endogenous Antigen
Present 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).

12. Function of Activated CD8 cell
Perforins: formation of channels
through the membrane.
Pore formation.
Cell content is lost.
Cell dies.

13. CD8 ( Cytotoxic T Cells)
Granzymes:
Proteases
Degrade protein in cell membrane loss
of cell content
Cell dies

14. Activation of CD8 (Cytotoxic T
Cells)

15. Function of Activated CD8 (Cytoxic
T Cells)