Hypersensitivity Reactions
Antigen-antibody reaction can be caused by one of two basic mechanisms:
- Allergic reactions with immediate response (Types I,II,III ) Humoral immune response stimulated by the sequence : B-Lymphocytes → plasma cells → immunoglobulins.
- Allergic reactions with delayed response ( Type IV ) Cell – mediated immune response stimulated by T – lymphocytes.
| REACTION TYPE | ANTIGEN | ANTIBODY |
|---|---|---|
| Anaphylactic type Type I |
Dust, feathers,Pollen, foodstuffs, Drugs |
Anaphylactic antibodies of class IgE |
| Cytotoxic Type (Type II) |
Cell membrane fragments or antigens (haptens) deposited on cell Cell |
Cytotoxic antibodies of classes IgG ,IgM,IgA |
| Arthus reaction (Type III) |
Foreign serum drugs, antigens from streptococci and tumors |
Precipitating antibodies of class IgG plus complement |
| Cell-mediated defense ( Type IV ) 1.Eczematous type 2.Tuberculin type |
Chemicals and noxious substances of different kinds |
T-lymphocytes |
Immunological Reactions and Effects
| Reaction Mechanism | Effects | Clinical manifestation |
|---|---|---|
| The reaction of IgE with the antigens causes degranulation of the mast cells, followed by release of histamine serotonin, bradykinin and SRS-A |
Vaso-dilation, increase of vascular permeability, contraction of smooth muscle |
Urticaria, angioedema, anaphylactic shock, Bronchial asthma, Allergic rhinitis |
| Cellular antigens react with circulating antibodies, or cellular antibodies react with circulating antigens. Complement becomes attached to the antigen-antibody complex, leading to lysis of the target cell. |
Cytolysis | Thrombocytopenia agranulocytosis hemolytic anemia, autoimmune disease |
| Deposition of antigen-antibody Complexes in capillary walls. Activation Of the complement system. Leukotaxis And phagocytosis of the immune Complexes by the leukocytes. Lysis of The phagocytosing cells and destruction Of the cell walls by lysosomal enzymes |
Inflammation, necrosis |
Arthus, phenomenon, serum sickness , drug rash toxic epidermal necrolysis, erythema multiforum erythema nolosum |
| As a result of the lymphocyte-antigen reaction humoral factors ( lymphokines ) are released Allergens invade the epidermis (contact Dermatitis) Allergens invade the dermis (tuberculin Type ) |
Vasodilation edema, monolympho cytic infiltration |
Allergiccontact dermatitis, allergic photo-dermatitis, allergic Exanthems,id reactions, Transplant rejection |
Hypersensitivity Reactions Of Irritants In Epidermis
In epidermal hypersensitivity reactions, the irritants directly the affect epidermis
| Irritant and allergic contact dermatitis |
Direct injury to skin caused by toxic chemicals (irritant contact dermatitis) or by substances which, though not primarily toxic, affect persons previously sensitized to the contact allergen. |
| Intertrigo | Toxic injury to skin caused by sweat and metabolic Products of saprophytic bacteria. |
| sunburns | Skin damage caused by exposure to sun light |
| phototoxic and photo-allergic dermatities |
Skin damage caused by psoralen (furocoumarin), and other photoxic or photo-allergic agents and subsequent exposure to sunlight . |
| Nummular (bacterial ) dermatities | Skin damage due to bacterial antigens or bacterial toxins |
| Seborrheic dermatitis | Skin damage due to quantitative and qualitative changes in sebum and sweat secretion. |
| Atopic dermatitis | Skin damage due to constitutional factors ( chronic abnormal vegetative response). |
Hypersensitivity reactions of the skin with Vascular involvement
In this type of hypersensitivity reaction the irritant Irritant affects both the cutis and the blood vessels.
| Urticaria angioedima |
Edematization of the subcutaneous tissue due to tissue due to allergic antigen –antibody reaction, mechanical irritation or toxic factors. |
| Anaphylactic shock | Massive disturbance of circulatory function due to generalized anaphylactic hypersensitivity reaction Of the immediate type ( Type ) . |
| Erythema multiforme | Reaction of diverse etiology which may be caused by bacterial or viral antigens or be drug-induced. Results in edematious saturation of the tissue and characteristic lesions. |
| Toxic and allergic Exanthems | Vascular hypersensitivity reaction caused by action of various toxic or allergenic substance ( especially drugs ) ; commonly manifests as a polymorphic eruption . |
| Allergic cutaneous Vasculitis |
An allergic-hyperergic reaction of the small vessels of the dermis associated with drug therapy or streptogenic infections . |