Immune System

• The immune system is a collection of cells, tissues, and organs that protect the body from invasion by foreign organisms and substances. The white blood cells, created in the bone marrow and circulating in the blood, perform different duties depending on their type. Types of white blood cells, called macrophages, engulf and digest bacteria and other foreign matter, like allergens and viruses. After digesting the foreign matter with enzymes and other chemicals, the macrophages present a piece of the foreign matter to another type of white blood cell called a lymphocyte. The piece that is presented is called an antigen. Lymphocytes use the antigen to create a corresponding antibody.
  
  

Antibodies

• IgM antibody. Notice the 5 variable regions (in orange)
  
  Antibodies, also called immunoglobulins, are protein molecules created by lymphocytes in response to being exposed to an antigen. Antibodies are very specific to the antigen to which they attach. For example, antigens against one strain of influenza may not attach to a different strain. Antibodies come in five classes: immunoglobulins (Ig) G, A, M, E and D. They also appear at different times after an infection and are specific to certain regions of the body.
  
  

Different Antibody Types

• IgA, notice the two variable regions (in orange)
  
  IgM is the first antibody to appear after an exposure to foreign matter. It is less specific than the other classes, but it is larger. It is a pentamere, meaning that five (penta-) sections on it are active. IgM does not circulate in blood too long after exposure. However, if exposure to the same antigen occurs again, IgM levels in the body will rise again (the "booster" effect). A few weeks to months after an exposure, IgG appears. It is a y-shaped protein molecule that is very specific to one antigen type. IgG is, in most cases, a life-long antibody, giving the body immunity from subsequent infections. IgA is more closely associated with body secretions more than the blood. It is the antibody found in tears and maternal milk. IgE is closely associated with allergic reactions. It is mostly found on skin and mucous membranes. IgD is the least specific antibody, usually found in the intestinal mucosa.
  
  

Variable and Constant Regions

• IgG antibody. Notice the traditional Y-shape.
  
  Antibodies have variable and a constant regions. The variable region is the region subject to change from one antibody to another, depending on the antigen for which they are created. It is coded by different genes in the lymphocytes. The constant region is the same in all antibodies. It can be regarded as the backbone of the antibody.
  
  

Functions

• How IgE antibodies trigger allergic symptoms
  
  Antibodies bind to their specific antigens and help eliminate those antigens from the body by tagging them for absorption by macrophages. When the foreign matter is within a cell, in the case of viruses and blood parasites, the cell will display an antigen to which the antibody will attach. This marks the cell for destruction so that the invading organism is also destroyed before it reproduces. In the case of allergies, IgE attaches to the allergen and to a white blood cell called a mast cell. The mast cell releases chemicals called histamines which are responsible for allergy symptoms like runny nose and watery eyes. The symptoms help spread IgA and IgE further to prevent more allergens from entering the body.