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Understanding the immune system.

To better understand immune deficiency, it may help to understand how the immune system works.

Components of the immune system.

The immune system represents a complex collaboration between different types of cells and proteins that protect against foreign invaders that cause infection. It is spread throughout the body, which enables it to quickly respond to the threat of infection regardless of where it occurs.

Parts of the immune system include the:

  • Blood vessels and lymphatic system, which transport immune cells throughout the body
  • Lymph nodes and spleen, where immune cells gather and communicate
  • Bone marrow, where immature immune cells are made
  • Thymus, where immature immune cells are “trained” to attack invading cells
  • Liver, which produces proteins and cells that ingest bacteria
  • Tonsils, which collect immune cells in the throat

Innate and adaptive immune response.

We can think of the immune system as having two parts: innate immune system and adaptive immune system.

Innate immune responses happen automatically – the cells don’t need to “learn” anything to mount an attack. This type of immune response happens very quickly and efficiently. Cells and proteins involved in innate immune responses include:

  • Neutrophils
  • Monocytes
  • Natural killer (NK) cells
  • Complement proteins

An adaptive immune response requires that cells “learn” to attack an invader without attacking the body’s own cells. This gives the immune system the ability to adapt to new threats. This type of response doesn’t happen as quickly as an innate immune response. The cells involved in adaptive immune responses are T-cells and B-cells.

In the event of an infection, the innate immune system typically responds right away. This alerts the adaptive immune system, which may take several days to fully activate.

The innate immune system is present as soon as we are born. The adaptive immune system, on the other hand, requires that we be exposed to infection so our cells can be “trained” to respond. With each infectious exposure, the immune system creates antibodies, proteins that allow the immune system to recognize an invader when we’re exposed again. This is why young children get sick more often than adults – their adaptive immune systems haven’t yet learned enough to respond to invaders that cause infections. Each time they are exposed to virus or bacteria, their immune response will become stronger.

Mounting an immune response.

The immune system responds to two main threats: the viruses and bacteria that we’re exposed to every day. The immune system responds to each of these differently.

Bacteria can invade the body when our natural defenses, the skin and mucous membranes, are compromised by injury or inflammation. For example, bacteria are always on the surface of our skin – a cut or scrape can allow the bacteria to get into the body. When the bacteria get in, they are coated with proteins that make them visible to neutrophils, which engulf and destroy them.

When this process works well, the body responds to and eliminates the threat quickly. If the system becomes overwhelmed by the number of bacteria, or if there is a defect in the system, recurrent bacterial infections can happen.

Viruses are different in that they enter cells and reproduce inside them. When a cell is invaded by a virus, it releases a protein called a cytokine to alert the immune system of the threat. Sometimes, this is enough to stop the infection. However, some viruses have developed the ability to hide from the immune system, which allows the infection to spread.

Once alerted to the presence of a virus, T-cells and NK cells work to destroy the virus. Unfortunately, many of the body’s own cells are destroyed in this process, which is why we feel the symptoms of an illness. While they are engaged in destroying the invading viruses, they are also instructing B-cells to create antibodies. These antibodies will help prevent infection if we’re exposed a second time.

In people with immune deficiency, the immune system is not able to carry out this process effectively because one or more components of the immune system is missing or not functioning properly. This makes the body more susceptible to infections.

If you have questions, talk to your doctor or contact your CVS Specialty CareTeam.

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