Immunity

Subject: Science and tech

Context:

Two recent studies have confirmed that people previously infected with SARS-CoV-2, the virus that causes COVID-19, can be reinfected with the virus.

Concept:

• Any infection initially activates a non-specific innate immune response, in which white blood cells trigger inflammation. This may be enough to clear the virus.
• But in more prolonged infections, the adaptive immune system is activated. Here, T and B cells recognise distinct structures (or antigens) derived from the virus.
• T cells can detect and kill infected cells, while B cells produce antibodies that neutralise the virus.
• During a primary infection – that is, the first time a person is infected with a particular virus – this adaptive immune response is delayed. It takes a few days before immune cells that recognise the specific pathogen are activated and expanded to control the infection.
• Some of these T and B cells, called memory cells, persist long after the infection is resolved. It is these memory cells that are crucial for long-term protection. In a subsequent infection by the same virus, the memory cells get activated rapidly and induce a robust and specific response to block the infection.
• A vaccine mimics this primary infection, providing antigens that prime the adaptive immune system and generating memory cells that can be activated rapidly in the event of a real infection.
• However, as the antigens in the vaccine are derived from weakened or noninfectious material from the virus, there is little risk of severe infection.