Review Of Immunity And Allergy

The two types of acquired immunity, depending on the primary immune-cell response, are:

1. The humoral response primarily involves production of antibodies from B cells. Of the five major classes of immunoglobulin (Ig)—IgA, IgD, IgE, IgG, and IgM. IgE

typically responds to parasitic infections and is the prime antibody that provokes immediate hypersensitivity allergic reactions (the majority of clinically diagnosed food allergies). IgA, IgD, IgG, and IgM antibodies are typically involved in longer-term immunologic processes; IgG is the largest portion of the 80% of total circulating antibodies. Of the five classes of immunoglobulins, IgG, IgM, and IgE are known to be involved in hypersensitivity reactions.

2. Cell-mediated immune responses typically involve destruction of infected cells by cytotoxic T cells or destruction of intracellular pathogens by macrophages activated by Th1 cells. Th1 and Th2 cells can also contribute to humoral immunity by up-regulating production of IgA, IgE, and the four subclasses of IgG.

The mechanisms of immune-mediated tissue injury and disease fall into four major categories:

1. Type I immediate hypersensitivity—These IgE-mediated reactions (e.g., allergic rhinitis, asthma, anaphylaxis, in the clinical subgroups atopy and anaphylaxis) occur within minutes with late-phase inflammatory responses that may occur hours later. The reactions involve vasodilation, smooth-muscle contractions, and mucous-gland secretions.

2. Type II antibody-mediated (cytotoxic) hypersensitivity—These reactions (e.g., hemolytic anemia, Rh-factor hemolytic disease) involve specific reactions of IgG or IgM to cellular antigens and include activation of a complement cascade and cell destruction.

3. Type III immune complex-mediated hypersensitivity—These reactions (e.g., serum sickness, Arthus reactions) involve IgG or IgM forming complexes with allergens to activate a complement, resulting in a rise in inflammatory mediators.

4. Type IV T-cell-mediated hypersensitivity (delayed hypersensitivity)—These reactions (e.g., contact dermatitis, tuberculin reactions) are mediated by T-helper lymphocytes and cytotoxic T cells, not by antibodies, and occur when contact with an allergen leads to dermal inflammation, with a latency period of one to two days after contact.

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