Summary

15.1 Hypersensitivities

• An allergy is an adaptive immune response, sometimes life-threatening, to an allergen.
• Type I hypersensitivity requires sensitization of mast cells with IgE, involving an initial IgE antibody response and IgE attachment to mast cells. On second exposure to an allergen, cross-linking of IgE molecules on mast cells triggers degranulation and release of preformed and newly formed chemical mediators of inflammation. Type I hypersensitivity may be localized and relatively minor (hives and hay fever) or system- wide and dangerous (systemic anaphylaxis).
• Type II hypersensitivities result from antibodies binding to antigens on cells and initiating cytotoxic responses. Examples include hemolytic transfusion reaction and hemolytic disease of the newborn.
• Type III hypersensitivities result from formation and accumulation of immune complexes in tissues, stimulating damaging inflammatory responses.
• Type IV hypersensitivities are not mediated by antibodies, but by helper T-cell activation of macrophages, eosinophils, and cytotoxic T cells.

15.2 Autoimmune Disorders

• Autoimmune diseases result from a breakdown in immunological tolerance. The actual induction event(s) for autoimmune states are largely unknown.
• Some autoimmune diseases attack specific organs, whereas others are more systemic.
• Organ-specific autoimmune diseases include celiac disease, Graves disease, Hashimoto thyroiditis, type I diabetes mellitus, and Addison disease.
• Systemic autoimmune diseases include multiple sclerosis, myasthenia gravis, psoriasis, rheumatoid arthritis, and systemic lupus erythematosus.
• Treatments for autoimmune diseases generally involve anti-inflammatory and immunosuppressive drugs.

15.3 Cancer Immunobiology and Immunotherapy

• Cancer results from a loss of control of the cell cycle, resulting in uncontrolled cell proliferation and a loss of the ability to differentiate.
• Adaptive and innate immune responses are engaged by tumor antigens, self-molecules only found on abnormal cells. These adaptive responses stimulate helper T cells to activate cytotoxic T cells and NK cells of innate immunity that will seek and destroy cancer cells.
• New anticancer therapies are in development that will exploit natural adaptive immunity anticancer responses. These include external stimulation of cytotoxic T cells and therapeutic vaccines that assist or enhance the immune response.