Figure 1. The measles virus. Source: https://images.fineartamerica.com/images-medium-large-5/measles-virus-particle-alfred-pasieka.jpg

Measles Transmission

Measles Symptoms

Immune Amnesia: How Your Immune System Forgets to Fight

How is immune amnesia accomplished?

How long does the amnesia last?

What can be done to correct or prevent the problem?

How Does Measles Virus Cause Immune Amnesia?

The risk associated with measles infection is much greater than the sum of its observable symptoms. The immune memories that you have acquired are priceless, built over many years and from countless exposures to a menagerie of germs. Measles virus is especially dangerous because it has the ability to destroy what’s been earned: immune memory from previous infections. Meanwhile, the process of fighting measles infection leaves patients especially vulnerable to secondary infection. The worldwide increase in measles prevalence is cause for concern because morbidity and mortality from the disease extends far beyond acute measles infection. How exactly can one virus wreak so much havoc?Perhaps the best-known characteristic of measles is its extreme contagiousness . The measles virus (MV), a single-stranded (-)RNA virus that belongs to the genus morbillivirus, has only one natural host: humans. MV is extremely transmissible by aerosol droplets; in a room full of exposed people, 90% who are unvaccinated will develop the disease. To complicate matters, this microbe can linger in the air for up to 2 hours.Infected individuals show symptoms 10-12 days after exposure, including fever, cough, runny nose, watery eyes, Koplik spots , and rash. Typically, measles infection is self-limiting and requires nothing more than palliative care to treat these symptoms. Immune-mediated clearance of the virus results in recovery and life-long immunity to the disease.More serious complications such as pneumonia, encephalitis, and even death can occur during acute infection. Statistics show that 2-3 out of every 1000 cases will result in brain damage or death. In 2017 alone, the WHO estimated that measles and its related secondary infections were responsible for approximately 110,000 deaths worldwide. Most of these were in children less than 5 years of age.One of the most unique—and most dangerous—features of measles pathogenesis is its ability to reset the immune systems of infected patients. During the acute phase of infection, measles induces immune suppression through a process called immune amnesia. Studies in non-human primates revealed that MV actually replaces the old memory cells of its host with new, MV-specific lymphocytes. As a result, the patient emerges with both a strong MV-specific immunity and an increased vulnerability to all other pathogens.Many pathogens suppress immune function; the influenza virus damages airway epithelial cells and increases patient susceptibility to pneumonia-causing bacterial species. However, the ability to destroy immunological memory and replace memory lymphocytes is unique to MV. This MV-specific phenomenon raises a number of questions:MV causes infection by fusing with the plasma membranes of host cells in a receptor-dependent manner. When MV enters the respiratory tract, it infects alveolar macrophages in the lungs first. The primary role of these specialized immune cells is to engulf and destroy foreign substances like dust, bacteria, and viral particles. Alveolar macrophages also possess a membrane glycoprotein called Signaling Lymphhocytic Activation Molecule (SLAM) that has been identified as the high affinity cellular receptor for MV . MV uses SLAM to fuse directly with the plasma membrane, bypass destructive phagocytosis, and release its genome and replication machinery directly into the cell cytoplasm. Instead of destroying the measles virus upon contact, hijacked macrophages transport viral copies straight to the closest lymph nodes for dissemination.Infected macrophages travel to lymph tissue, where the virus comes in contact with the memory cells of the immune system memory T-cells and B-cells ). These lymphocytes are recon strategists. They identify foreign invaders through antigen detection and process these molecular patterns to generate long-lived memory cells for future protection. If a second encounter occurs, memory cells will mount a faster and stronger immune response to that pathogen than during the first encounter.

Figure 2. The measles infectious life cycle shows an important role for alveolar macrophages and lymphocytes in the course of disease. Source: http://www.immunopaedia.org.za/wp-content/uploads/2014/12/measles-virus-infection-cycle.jpg

Memory T-cells and B-cells contain SLAM surface receptors as well. Research has shown that MV binds and infects memory T-cells, memory B-cells, and naive B-cells of the immune system. Once infection is established, the virus spreads through the body by budding from infected cells. Clearance of MV requires the elimination of virally-infected lymphocytes. Immune-mediated destruction of memory T cells and B cells is initiated, and memories of past infections are destroyed along with them.



The number of T cells and B cells significantly decreases during the acute stage of measles infection, but there is a rapid return to normal WBC levels after the virus is cleared from the system. This observation masked what was really going on until researchers were able to evaluate the qualitative composition of recovered lymphocyte populations. We now know that the memory T-cells and B-cells that are produced immediately following infection are dramatically different from those that existed before the measles infection. Not only have pre-existing immune memory cells been erased, but there has been a massive production of new lymphocytes. And these have only one memory. Measles. Thus, the host is left totally immune to MV and significantly vulnerable to all other secondary infections. But for how long?

We now know that the memory T-cells and B-cells that are produced immediately following infection are dramatically different from those that existed before the measles infection.

How Long Does the Amnesia Last?

What Can Be Done to Correct or Prevent the Problem?

Why Measles Vaccination Matters

Figure 3. An infant with the typical measles rash. Source: http://cliniquecme.com/wp-content/uploads/2019/01/baby-measles-vaccine.jpg