The immune system is a complicated system of specialized cells and organs throughout the human body designed to recognize foreign substances (antigens) and react to them. Antigens commonly include toxins, viruses, bacteria, fungi, parasites and cancer cells, although they can also include transplanted cells or tissues. Immune system reactions usually result in the antigen being turned into a less dangerous substance or being removed from the body entirely.  

Sometimes the immune system will mistake a harmless substance (such as pollen or a protein in peanuts) as a threat, triggering an allergic reaction. Allergies and allergy symptoms are a result of the immune system’s response to allergens (see Response to allergens).

The immune system is spread throughout the human body, which allows it enough access to effectively protect against invaders in almost every part of the anatomy. With a variety of components all acting together, the immune system is capable of rapidly overcoming most invaders. Many of these components operate at the cellular level. These include:

• White blood cells. While there are many different types of white blood cells, all of them work together to destroy bacteria, fungi, viruses and other harmful substances in the bloodstream.  All white blood cells are called leukocytes, and each one is capable of moving and acting independently to defend the body against invaders. White blood cells are divided into three different classes:
  
  
• • Granulocytes. Made up of basophils, eosinophils, and neutrophils.
    
    
  • Lymphocytes. Made up of T-cells and B-cells:
    
    
  • • T-cells. This type of cell comes in two forms: Killer T-cells are capable of distinguishing between foreign and friendly cells on their own, directly attacking invaders when they are detected. Helper T-cells are used to stimulate B-cells.
      
      
    • B-cells. These types of cells need to be stimulated by helper T-cells before they can respond. However, once they are stimulated, they are capable of tuning in to a specific germ or antigen. They then begin cloning themselves and produce millions of antibodies designed to attack that specific antigen.
      
      
  • Monocytes. Evolve into macrophages (scavenger cells that remove foreign bodies from the blood).
    
    
• Antibodies. Produced by white blood cells, these proteins are able to defend against dangerous substances by responding to specific antigens (e.g. bacteria, viruses). Antibodies are shaped like the letter Y, with the two branches sensitized to bind together with an antigen if it is encountered. This binding action usually disrupts the chemical action of the invader.  The five classes of antibodies are:
  
  
• • Immunoglobulin A (IgA)
  • Immunoglobulin D (IgD)
  • Immunoglobulin E (IgE)
  • Immunoglobulin G (IgG)
  • Immunoglobulin M (IgM)
    
    
• Hormones. The hormones generated by the immune system are called lymphokines. Some of these hormones are used by the body to either suppress or encourage other parts of the immune system.
  
  
• Complement system. This series of proteins floats freely in the blood and works together with antibodies to either cause cellular bursting or signal that a certain cell needs to be removed from the body.

A number of organs and tissues are involved in the immune response by manufacturing or regulating the types of cells present in the bloodstream. These include:

• Bone marrow. Capable of producing new red and white blood cells, bone marrow is vitally important to the immune system. Marrow is located in the cavities of the bones and produces new blood cells from stem cells, which are capable of becoming many different types of cells.
  
  
• Thymus. Located in the front of the chest at the base of the neck, this organ is most important to infants, who depend on it for the maturation of T–cells. Adults can usually make enough T–cells elsewhere in the body that the thymus is not an essential organ.
  
  
• Spleen. The spleen is an organ located under the rib cage on the left side of the abdomen. It contains macrophages (scavenger cells that remove foreign bodies from the blood).
  
  
• Lymphatic system. A network of nodes that extends throughout the body. Lymph (blood plasma that bathes cells in water and nutrients) is distributed through the system to provide cells with the food, water and oxygen they need to survive.
  
  
• Adenoids and tonsils. These masses of lymphoid tissue are located in the back of the throat. They store white blood cells that produce antibodies against foreign materials that are breathed in or swallowed.