Major Histocompatibility Complex and Hypersensitivity Lecture Notes

Flashcards covering the Major Histocompatibility Complex (MHC), HLA types, transplantation, graft rejection, and the four types of hypersensitivity reactions.

Major Histocompatibility Complex (MHC)

A gene region located on the short upper arm of chromosome 6 in humans that encodes antigen-presenting molecules designated as Class I and Class II.

Human Leukocyte Antigen (HLA)

The name given to the MHC in humans, originally discovered in leukocytes but present in all nucleated cells.

MHC Class I

Molecules comprised of a polymorphic heavy chain and a non-polymorphic light chain ($\beta_2$-microglobulin), present in all nucleated cells.

MHC Class II

Molecules comprised of two polypeptide chains ($\alpha$ and $\beta$) found specifically on Antigen Presenting Cells (APCs).

MHC Class III

A portion of the MHC gene region that encodes for proteins of the complement system.

HLA-A, HLA-B, HLA-C

The specific genetic denotations used to identify Class I MHC molecules.

HLA-DP, HLA-DQ, HLA-DR

The specific genetic denotations used to identify Class II MHC molecules.

CD8+ T cells

Cytotoxic T cells that only recognize antigens presented by Class I MHC molecules.

CD4+ T cells

Helper T cells that only recognize antigens presented by Class II MHC molecules.

Codominant Expression

The inheritance pattern of MHC molecules where both maternal and paternal alleles are expressed simultaneously on the cell surface.

HLA B27

A specific HLA type strongly correlated with diseases such as ankylosing spondylitis and Reiter’s syndrome.

Isograft

A type of transplant where tissues are transferred from one part of the body to another part within the same individual.

Allograft

A transfer of organs or tissues between different individuals of the same species.

Xenograft

A graft or transplant performed between individuals of different species.

Hyperacute Rejection

A graft rejection occurring within minutes to hours after transplant, primarily mediated by antibodies.

Acute Rejection

A graft rejection occurring weeks after tissue transplant, primarily mediated by T cells and antigen-presenting cells.

Chronic Rejection

A graft rejection occurring weeks, months, or years after transplant, derived from specific growth factors in the immune system.

Graft vs. Host Disease (GVHD)

A condition occurring after bone marrow transplant where the donor's immune cells attack the recipient's tissues.

Mixed Lymphocyte Reaction (MLR)

A cellular histocompatibility test where donor and recipient cells are cultured together to observe T cell activation and proliferation.

Hypersensitivity

A normal but exaggerated immune response to an antigen that produces inflammation, cell destruction, or tissue injury.

Atopy

Immediate hypersensitivity mediated specifically by IgE antibodies.

Type I Hypersensitivity

An anaphylactic, IgE-mediated immediate reaction where the primary mediator is histamine.

Anaphylactoid Reaction

A clinical response similar to anaphylaxis that is not mediated by IgE but by substances acting directly on mast cells.

Type II Hypersensitivity

A cytotoxic reaction resulting from IgG or IgM binding to cell surfaces, involving complement or NK cells.

Type III Hypersensitivity

An immune complex-mediated reaction where antigen-antibody complexes deposit in blood vessel walls and tissues, activating the complement system.

Type IV Hypersensitivity

A T-cell dependent, delayed-type hypersensitivity (DTH) that does not involve antibodies, typically observed in TB skin tests or contact dermatitis.

Induration

A lesion of $\ge 10\,mm$ observed 48-72 hours after a TB skin test, indicating a positive Type IV hypersensitivity reaction.