Immunology Key Concepts: MHC, T Cells, Antibodies, Hypersensitivity, and Clinical Applications

MHC I

Presents endogenous peptides to CD8 cytotoxic T cells for intracellular pathogen surveillance.

MHC II

Presents exogenous peptides to CD4 helper T cells to coordinate immune responses.

Cross presentation

Dendritic cell ability to load exogenous antigens onto MHC I to prime CD8 T cells.

Costimulation

Second activation signal for T cells involving CD28 and B7 that prevents anergy.

Clonal selection

Expansion of lymphocytes whose receptors recognize a specific antigen.

Somatic hypermutation

Mutation process in germinal centers that increases antibody affinity.

Class switch recombination

Change in antibody constant region to IgG IgA or IgE while preserving specificity.

Neutralization

Antibody binding blocks pathogen attachment or toxin effects.

Opsonization

Coating of pathogens by antibodies or complement to enhance phagocytosis.

Membrane attack complex

Complement complex C5b 9 that forms pores and lyses susceptible microbes.

Perforin

Cytotoxic protein that forms pores in target membranes to deliver granzymes.

Granzymes

Serine proteases that trigger apoptosis in target cells.

Regulatory T cell

T cell subset that suppresses immune activation and maintains tolerance.

Th1

Helper T subset that produces IFN gamma to activate macrophages and support cell mediated immunity.

Th2

Helper T subset that produces IL 4 and IL 5 to promote IgE and eosinophils.

Th17

Helper T subset that produces IL 17 to recruit neutrophils at barriers.

Tfh

Helper T subset that provides B cell help in germinal centers.

Plasma cell

Antibody secreting effector cell derived from activated B cells.

Memory B cell

Long lived cell that responds rapidly upon re exposure to antigen.

IgM

First antibody produced during primary response strong complement activator.

IgG

Most abundant antibody in serum crosses the placenta and has versatile functions.

IgA

Dimeric antibody in mucosal secretions that protects epithelial surfaces.

IgE

Antibody that binds mast cells mediates type I hypersensitivity and defends against helminths.

Type I hypersensitivity

IgE mediated immediate reaction leading to allergy and anaphylaxis.

Type II hypersensitivity

Antibody mediated cytotoxicity against cell or matrix antigens causing complement activation.

Type III hypersensitivity

Immune complex deposition that inflames tissues as in lupus.

Type IV hypersensitivity

Delayed T cell mediated response such as contact dermatitis and the tuberculin test.

Autoimmunity

Immune response against self antigens resulting in tissue damage and chronic disease.

SCID

Primary immunodeficiency with severe T and B cell defects causing recurrent infections.

Herd immunity

Population level protection when a high proportion of individuals are immune.

Checkpoint blockade

Therapy that blocks inhibitory receptors like PD 1 or CTLA 4 to restore antitumor T cell activity.

CAR T cell

Engineered T cell that targets specific tumor antigens for potent killing.

Clinical tie transfusion reaction

Type II hypersensitivity where recipient antibodies lyse donor red blood cells via complement.

Clinical tie TB skin test

Type IV delayed hypersensitivity where memory T cells cause induration at 48 to 72 hours.

Clinical tie anaphylaxis treatment

Immediate epinephrine to counteract mast cell mediators and support airway and circulation.

Clinical tie lupus nephritis

Type III immune complex deposition in glomeruli leading to inflammation and renal dysfunction.

Clinical tie vaccine booster

Secondary response produces faster higher affinity IgG titers than the primary response.