The process by which the immune system deals with antigens is complicated. The body is capable of defending itself in many different ways and in many different places. Just getting into the bloodstream is difficult for antigens. Some of the very first processes through which the immune system defends the body include:

• External protection. The first and most basic layer of the immune system is the skin. While not a true immune system component, skin is able to function as a first line of defense against antigens, keeping many antigens out of the body entirely and providing an early warning system for those that manage to penetrate it. Skin prevents bacteria and viruses from passing, and actually secretes antibacterial substances to prevent bacteria and spores from growing on the skin.
  
  
• Entrances to the body. While the nose, mouth and eyes are gateways to the inner body, they are also part of the first line of defense against antigens. All three use mucus and enzymes to combat and keep out many types of invaders. Antigens frequently become trapped in mucus, and are removed by the body through actions such as coughing and sneezing. The nose and airways also contains cilia (hair-like projections) which sweep mucus up and away from the lungs.
  
  

  

• Boundary tissues. Before an antigen can enter the bloodstream, it needs to make it past the bodies various boundary tissues. Macrophage cells live in specific areas, such as the lining of the lungs. When they come into contact with a foreign particle, they help to neutralize it by moving it out of the bloodstream. A form of macrophages (Langerhans cells) live in the skin, where they fight against any invaders that could otherwise be absorbed through the skin and into the bloodstream.

Those antigens that succeed in making it into the bloodstream quickly find themselves identified as dangerous. Several types of cells known as antigen presenting cells (APCs) are located throughout the body, constantly checking their surrounding environment for any foreign antigens. When an antigen is encountered by the APCs, the cells actually break down the molecules of the antigen and display its components. This action makes any antigen easily identifiable to those parts of the immune system tasked with attacking foreign substances.

With the help of the APCs, neutrophils (the most common type of white blood cells) are able to quickly recognize dangerous invaders and release enzymes to kill them. Antibodies can also identify many types of antigens, and will attach themselves to invaders to disrupt them. Antibodies and some types of leukocytes (white blood cells) will trigger chemical responses, such as the release of histamines or leukotrienes, which make the body less hospitable for the invading antigen.

Most of the attacking cells that make up the immune system are able to distinguish between friendly local cells and foreign cells through the major histocompatibility complex (MHC). This process actually marks, through genes in protein molecules, every individual cell normally found in the body as friendly. When the immune system encounters a cell that is not identified as part of the MHC, it reacts against it.

When dealing with an antigen invader, the immune system will often produce symptoms that people find unpleasant. Runny nose, itching, swelling and coughing are all common immune system responses to antigens. While the onset of these symptoms might make it seem like the immune system is malfunctioning, it is actually producing these symptoms for very good reasons.

For example, the immune system favors inflammation and swelling because they dilate capillary walls (through which blood flows) and increase blood flow to an infected area. This allows more immune system cells to get to the point of infection faster.