TABLE Characteristics of Human Immunoglobulins14.2 Isotype Molecular Weight (kDa) Serum Level (mg/mL) Percentage of all Ig in Adult Blood Cells to Which Bind via Fc Region Major Functions IgG 145 12.0 85 Macrophages, B cells, NK cells,  neutrophils, eosinophils Principal Ig in secondary immune response Longest half-life (23 days) of all five Igs Activates complement Stimulates chemotaxis Crosses placenta,  providing newborn with passive immunity IgM 190 (950)a1.5 5–10 B cells Principal Ig produced during primary immune response Most efficient Ig in fixing complement Activates macrophages Serves as Ag receptor of B lymphocytes IgA 160 (385)b2.0 5–15 B cells Ig present in body secretions, including tears, colostrum, saliva, and vaginal fluid, and in secretions of nasal cavity, bronchi,  intestine, and prostate Provides protection against proliferation of microorganisms in these fluids and aids in defense against microbes and foreign molecules penetrating body via cell linings of these cavities IgD 185 0.03 1 B cells Acts as an antigen receptor (together with IgM) on surface of mature B lymphocytes (only traces in serum) IgE 190 3 × 10 5 1 Mast cells, basophils Stimulates mast cells to release histamine, heparin, leukotrienes, and eosinophil chemotactic  factor of anaphylaxis Responsible for anaphylactic hypersensitivity reactions Increased levels in parasitic infections a IgM found in serum as a pentameric molecule. b IgA found in serum as dimeric molecule. Ag, antigen; Ig, immunoglobulin; NK, natural killer.
response because of the presence of specific memory B lymphocytes already programmed to respond to that specific antigen. The secondary response is the basis of most immunizations for common bacterial and viral diseases. Some antigens, such as penicillin and insect venoms, may trigger an intense secondary immune response that produces a hypersensitivity reaction or even anaphylaxis (see Folder 14.2). However, antibodies themselves do not kill or destroy invading antigens; they simply mark them for destruction by cells of the immune system.
The two types of specific immune responses are the humoral and cell-mediated responses.
In general, an encounter with a given antigen triggers a response characterized as either a humoral immune response (antibody production) or a cell-mediated immune response.
 FOLDER 14.2 Clinical Correlation: Hypersensitivity Reactions
When an individual has been immunologically sensitized by exposure to antigen, subsequent exposure may lead not only to secondary response but also to tissue-damaging reactions called hypersensitivity reactions. Such reac-tions are observed in sensitized humans after insect bites or injections of penicillin. One common type of hypersen-sitivity reaction is the allergic reaction. Certain aspects of a hypersensitivity reaction are caused by the antibody-induced discharge of mast cell granules. These granules contain histamine, which accounts for the distressing fea-tures of hypersensitivity reactions. Eosinophils are  FOLDER 14.1 Functional Considerations: Origin of the Names T Lymphocyte and B Lymphocyte