Hypersensitivity Definition:

Hypersensitivity refers to an excessive or exaggerated immune response triggered by exposure to substances typically harmless to most individuals. This abnormal reaction can occur through various mechanisms and is categorized into four types, known as types I, II, III, and IV hypersensitivity reactions.

Type I Hypersensitivity:

Type I hypersensitivity, also referred to as immediate hypersensitivity, is an allergic reaction mediated by immunoglobulin E (IgE) antibodies. It is typically rapid and occurs within seconds to minutes after exposure to the allergen. Common examples include allergies to pollen, dust mites, pet dander, and certain foods.

Type II Hypersensitivity:

Type II hypersensitivity involves the activation of complement and binding of antibodies, such as immunoglobulin G (IgG) or immunoglobulin M (IgM), to antigens on cell surfaces. This reaction can lead to cell destruction through mechanisms such as antibody-dependent cell-mediated cytotoxicity (ADCC) or complement-mediated lysis. Examples include autoimmune diseases like autoimmune hemolytic anemia and certain drug reactions.

Type III Hypersensitivity:

Type III hypersensitivity occurs when immune complexes comprising antigens and antibodies (mainly IgG and IgM) deposit in tissues and activate the complement system. This activation leads to inflammation and tissue damage. Diseases associated with type III hypersensitivity reactions include systemic lupus erythematosus, rheumatoid arthritis, and certain forms of vasculitis.

Type IV Hypersensitivity:

Type IV hypersensitivity, also called delayed-type hypersensitivity (DTH), is a cell-mediated immune response involving primarily T lymphocytes. Unlike the immediate hypersensitivity reactions seen in other types, type IV reactions typically take hours to develop and peak within 48-72 hours. Examples of type IV hypersensitivity reactions include contact dermatitis, tuberculin skin tests, and graft rejection.

It is important to note that hypersensitivity reactions can vary in severity and manifestations, ranging from mild allergic symptoms to life-threatening conditions like anaphylaxis. Prompt recognition, appropriate management, and avoidance of triggers are crucial in the treatment and prevention of hypersensitivity reactions.