Immunity and Hypersensitivity

Immunity is the ability of the body to recognize and defend itself against foreign substances or organisms that may cause disease or harm. It involves two main types: innate immunity and adaptive immunity. Innate immunity is the first line of defense that provides immediate and nonspecific protection against any potential invader. Adaptive immunity is the second line of defense that provides delayed and specific protection against a particular invader. Some examples of immunity are:

• Innate immunity: consists of physical barriers (skin, mucous membranes), mechanical barriers (cilia, coughing, sneezing), chemical barriers (acid, enzymes, lysozyme), cellular barriers (natural killer cells, phagocytes), and inflammatory barriers (complement, cytokines, acute phase proteins). It does not require prior exposure to an antigen (a foreign substance that elicits an immune response) and does not generate immunological memory (a long-lasting response to a specific antigen).

• Adaptive immunity: consists of humoral immunity (mediated by antibodies produced by B lymphocytes) and cell-mediated immunity (mediated by T lymphocytes). It requires prior exposure to an antigen and generates immunological memory. It can be further classified into active immunity (acquired by natural infection or vaccination) or passive immunity (acquired by maternal transfer or immunoglobulin injection).

Hypersensitivity is an exaggerated or inappropriate immune response to an antigen that results in tissue damage or disease. It involves four main types: type I, type II, type III, and type IV. Some examples of hypersensitivity are:

• Type I hypersensitivity: also known as immediate hypersensitivity or allergic reaction. It is mediated by IgE antibodies that bind to mast cells and basophils and trigger the release of histamine and other mediators that cause vasodilation, increased vascular permeability, smooth muscle contraction, mucus secretion, and itching. It occurs within minutes after exposure to an antigen and can range from mild (urticaria, rhinitis, conjunctivitis) to severe (anaphylaxis).

• Type II hypersensitivity: also known as antibody-mediated hypersensitivity or cytotoxic reaction. It is mediated by IgG or IgM antibodies that bind to antigens on the surface of target cells and activate complement or phagocytes that destroy the target cells. It occurs within hours to days after exposure to an antigen and can affect various tissues and organs (hemolytic anemia, transfusion reaction, autoimmune hemolytic anemia, autoimmune hemolytic anemia, Graves’ disease, myasthenia gravis, Goodpasture syndrome).

• Type III hypersensitivity: also known as immune complex-mediated hypersensitivity or serum sickness. It is mediated by IgG or IgM antibodies that form antigen-antibody complexes that deposit in various tissues and activate complement or phagocytes that cause inflammation and tissue damage. It occurs within hours to days after exposure to an antigen and can affect various tissues and organs (glomerulonephritis, rheumatoid arthritis, systemic lupus erythematosus, Arthus reaction).

• Type IV hypersensitivity: also known as cell-mediated hypersensitivity or delayed-type hypersensitivity. It is mediated by T lymphocytes that recognize antigens on the surface of target cells and secrete cytokines or cytotoxic molecules that cause inflammation and tissue damage. It occurs within days to weeks after exposure to an antigen and can affect various tissues and organs (contact dermatitis, tuberculin reaction, graft rejection, type 1 diabetes mellitus, multiple sclerosis).