Epstein-Barr virus (Infectious mononucleosis)

The Epstein-Barr virus, or EBV, also known as human herpesvirus-4, or HHV-4, is an enveloped virus of the Herpesviridae family, which contains linear, double-stranded DNA.

It is the most common cause of infectious mononucleosis, more commonly known as “mono.”

EBV infection begins when saliva or respiratory secretions that contain the virus from one person make their way into someone else’s mouth.

Often, this happens by sharing food or drinks, or by kissing, which is why mono is sometimes called the “kissing disease” and why it is most common in adolescents and young adults, aged 15-24, because young people are often doing the smooching.

When EBV reaches the new person’s mouth, it infects two types of cells in the oropharynx: epithelial cells, which line the surfaces of the oropharynx, and B cells, which are lymphocytes, or lymphoid cells, that create antibodies to fight off infections.

In the epithelial cells, the virus undergoes the lytic cycle, whereby its DNA gets transcribed and translated by cellular enzymes, which help to form viral proteins, which are packaged into new viruses, which can leave the host cell destroyed, or lysed, and subsequently infect neighboring epithelial cells.

The viruses that reach the lymphoid tissue of the oropharynx, the tonsils, infect their main target, B cells, by attaching to their CD21 receptor, also known as the CR2 receptor.

The infected B cells then enter the latent phase, in which the virus just sort of hangs out in the host cell instead of killing it.

They then carry and spread the infection along their normal trajectory to the other lymphoid tissues of the body, including the liver, spleen, and lymph nodes.

The body’s immune system reacts to the infection by mounting a humoral response, whereby the B cells create antibodies to fight off the virus, and a cellular response, in which cytotoxic (or CD8+) T cells, which are lymphocytes that mature in the thymus, work to kill the infected B cells.