immune disorders
Acquired Immunity
Antibodies
When a B cell encounters an antigen, it is stimulated to mature into a plasma cell or a memory B cell. Plasma cells then release antibodies (also called immunoglobulins, or Ig). There are 5 classes of antibodies—IgM, IgG, IgA, IgE, and IgD. Antibodies protect the body in the following ways: Helping cells ingest antigens (cells that ingest antigens are called phagocytes) Inactivating toxic substances produced by bacteria Attacking bacteria and viruses directly Preventing bacteria and viruses from attaching to and invading cells Activating the Complement System , which has many immune functions Helping certain cells, such as natural killer cells, kill infected cells or cancer cells Antibodies are essential for fighting off certain types of bacterial and fungal infections. They can also help fight viruses. Antibodies attach to the antigen they were formed to recognize and form an immune complex (antibody-antigen complex). The antibody and antigen fit tightly together, like pieces of a jigsaw puzzle. Sometimes an antibody can attach to other antigens if the antigens closely resemble the antigen that the antibody was formed to recognize and attach to. An antibody molecule is basically shaped like a Y. The molecule has two parts: Variable part: This part varies from antibody to antibody, depending on which antigen the antibody targets. The antigen attaches to the variable part. Constant part: This part can be one of five structures, which determines the antibody’s class— IgM, IgG, IgA, IgE, or IgD. This part is the same within each class. Each antibody molecule has two parts: Variable part: This part varies. It is specialized to attach to a specific antigen. Constant part: This part is one of five structures, which determines the antibody’s class—IgM, IgG, IgA, IgE, or IgD. This part is the same within each class and determines the function of the antibody. An antibody can switch its constant part and become a different class, but its variable part does not change. Thus, it can always recognize the specific antigen that it was formed to attach to. This class of antibody is produced when a particular antigen (such as an antigen of an infectious microorganism) is encountered for the first time. The response triggered by the first encounter with an antigen is the primary immune response. IgM then attaches to the antigen, activating the complement system, and thus makes the microorganism easier to ingest. Normally, IgM is present in the bloodstream but not in the tissues. IgG, the most prevalent class of antibody, is produced when a particular antigen is encountered again. More antibody is produced in this response (called the secondary immune response) than in the primary immune response. The secondary immune response is also faster and the antibodies produced—mainly IgG—are more effective. IgG protects against bacteria, viruses, fungi, and toxic substances. IgG is present in the bloodstream and tissues. It is the only class of antibody that crosses the placenta from mother to fetus. The mother’s IgG protects the fetus and infant until the infant’s immune system can produce its own antibodies. Also, IgG is the most common class of antibody used in treatment. For example, immune globulin (antibodies obtained from the blood of people with a normal immune system) consists mainly of IgG. Immune globulin is used to treat some Overview of Immunodeficiency Disorders and These antibodies help defend against the invasion of microorganisms through body surfaces lined with a mucous membrane, including those of the nose, eyes, lungs, and digestive tract. IgA is present in the following: Bloodstream Secretions produced by mucous membranes (such as tears and saliva) Colostrum (the fluid produced by the breasts during the first few days after delivery, before breast milk is produced) These antibodies trigger immediate Overview of Allergic Reactions . IgE binds to Small amounts of IgE are present in the bloodstream and mucus of the digestive system. These amounts are higher in people with asthma, hay fever, other allergic disorders, or parasitic infections. IgD is present mainly on the surface of immature B cells. It helps these cells mature. Small amounts of these antibodies are present in the bloodstream. Their function in the bloodstream, if any, is not well understood. 