Sometimes a person’s immune system does not work properly. This can result from immune deficiencies present at birth; medications that suppress the immune system, like steroids; unnecessary or overzealous immune responses, such as allergies; or immune responses to one’s self, called autoimmunity. One of the amazing aspects of the immune system is that it is compensatory, meaning that when one part is weak or non-functional, typically another part can step in. Think of it like a trip to the grocery store. If you need to go to the store, but your tire is flat, you may go by another method of transportation — another motor vehicle, a bicycle or walking. The substitute may or may not be as efficient, but it still allows you to complete your task. The same is true of the cells and proteins of our immune system; most “jobs” of the immune system can be done by more than one part of the immune system although some parts are better at certain jobs than others.
This same feature that makes the immune system flexible also makes it difficult to study. This is why studies in the lab, and even in animals, still need to be repeated in people before we can be sure the findings apply. However, laboratory and animal studies remain important because they provide us with preliminary information that puts us in the best position to succeed when we complete studies in people.
All of this said, sometimes people still have conditions that alter their ability to respond to infections, so let’s take a look at a few and explore how the immune system works in these unique situations.
Immune deficiencies
Immune deficiencies can result from inherited or spontaneous genetic variations, from medications that suppress the immune system, or from infections that damage components of the immune system.
Genetic variations
A change to a person’s genes can result in the immune system lacking, or having non-functional, components. Most of these conditions are rare, but when they occur, a person is often diagnosed early in life because they experience a higher than average number of infections. More than 40 different deficiencies have been identified; a small number of examples include:
Severe combined immune deficiency (SCID), which results from problems with T cell development.
DiGeorge syndrome is a type of SCID, which results from improper T cell maturation.
X-linked agammaglobulinemia, which results in a lack of B cells.
Atypical hemolytic uremic syndrome, which is caused by a defect in certain immune-related proteins called complement. The result can be excessive swelling, and in some cases, the swelling can occur around the breathing tubes leading to suffocation.
Kostmann disease, also known as severe congenital neutropenia, which results in chronically low levels of the white blood cells known as neutrophils.
A group of periodic fever syndromes, which result from deficiencies in the pathway that controls inflammation, or swelling. Symptoms can include recurrent fevers, swelling or joint pain.
Medications
Medications like chemotherapies for cancer or immune suppressive medications for a variety of rheumatologic or allergic disorders.
Infections
Human immunodeficiency virus (HIV) is the most well-known example of a chronic immune system condition caused by an infection. HIV infects T cells, specifically a type of T cell called CD4+ T cells. This results in two issues. First, the immune response is severely compromised because, as described on the “Parts of the Immune System” page, T cells are the equivalent of police chiefs or sergeants, so the coordination of the immune response is hampered. The second issue is that as the immune system works to overcome the infection, it targets one of its own components.
HIV infections occur in phases that can be identified, in part, by a person’s CD4+ T cell count. Early in the infection, called the acute phase, CD4+ T cells decrease. The infected person may have influenza-like symptoms, but may not yet realize they are infected. However, if a person’s blood is tested during the acute phase, the virus can be detected at high levels. Initially, the T cell population rebounds, only to drop again over time. During this period, which can be short or last for years, the person is typically asymptomatic. Eventually, the CD4+ T cell population becomes so depleted that the individual starts to experience other, opportunistic, infections. This marks the beginning of the final phase, commonly known as acquired immune deficiency syndrome or AIDS, which eventually results in death. Often death is the result of one of these opportunistic infections.
