35. Transplantation antigens. Mechanisms of transplant rejection.

Question 1

Transplantation antigens

Answer 1

1. Blood Group Antigens:

-ABO System: Carbohydrate antigens on erythrocytes.

Antibodies:
-Group A has antibodies to group B.
-Group B has antibodies to group A.
-Transfusion: Incompatible blood is rejected.

2. MHC Antigens:
   -Function: Barrier to nucleated cell transplantation.
   -Location: Expressed on all nucleated cells.
   -Genes: Highly polymorphic, on chromosome 6 (HLA system).
   -Complexity: Six loci, multiple alleles; low match likelihood in unrelated individuals.
3. Minor Histocompatibility Antigens:
   -Nature: Non-MHC polymorphic antigens.
   -Rejection: Weaker/slower rejection.
   -Function: Presented with self MHC to T cells.

Question 2

Mechanisms of transplant rejection

Answer 2

Primary vs. Secondary Grafts:

Primary Graft: First transplant.
Allograft Rejection: Within 14 days (first set rejection).

Secondary Graft: Subsequent transplant from the same donor.

-Rejection: Within 5-6 days (second set rejection).
-Reason: Due to immunologic memory.

Rejection Mechanisms:
-Cause: Cell-mediated immune response to alloantigens, mainly MHC.
-Reactions: Delayed-type hypersensitivity and cytotoxicity.
-Stages:
-1.Sensitization: Lymphocytes proliferate in response to graft alloantigens.
-2.Effector: Immune destruction of the graft.

Prevention:
-Familial Grafting: Reduces HLA mismatches; higher match rates within families.
-Tissue Typing: Determines HLA compatibility using antibodies or genetic techniques.
-Cross-Matching: Tests recipient serum for antibodies against donor tissues.
-Immunosuppression: Uses drugs like corticosteroids and T-cell inhibitors to suppress immune response.