Have you ever caught the flu? And have you ever caught it more than once? If you have answered yes to these questions, you might be wondering how it is even possible to catch the flu on multiple occasions, since you may have heard it said that, once you have caught an illness, you develop an immunity to the pathogen. So why is this not the case for the flu, you ask? Well, you are not alone - scientists have been trying every year to create a vaccine to prevent each annual flu virus.
In order to understand why we can get the flu more than once, we must first understand how our immune system works. When a pathogen enters our body, white blood cells, known as phagocytes, engulf the foreign body, and then present the antigen. This is a chemical unique to each pathogen, that our immune system uses to recognise it. Presenting the antigen will activate T-cells, which in turn activate another type of white blood cell called a B-cell. These differentiate into two different types of cells: plasma cells, which attack the pathogen by producing specific antibodies, and memory cells. The memory B-cells stay in our blood until we die and protect us from the pathogen with the same antigen shape.
To answer why we are not immune to yearly flu: it is due to antigen variation of the influenza. The gene that codes for the influenza antigen can mutate by a process called "drift" or "shift."
A genetic drift is usually a small change in the shape of the antigen so that the two antigens are closely related to one another, as it is usually a product of a single base substitution in the genetic code. However, as the mutations accumulate, the antigen becomes so different that our immune system no longer recognizes it, which results in us getting the flu yet again.
Antigenic shift, meanwhile, is a major change in the shape of the influenza antigen. Usually, this only occurs in the influenza A subtype of the virus, when two or more strains co-infect a cell and exchange their genes in order to form a completely new strain. The mutation in the gene is very significant as the antigen becomes so different to the human influenza most people are not immune to it. Pathogens which have emerged from animal populations are also very foreign to a human immune system and can become seasonal flu too. This may arise by the exchange of genes between animal and human strains of flu. While it is unusual for people to get influenza infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses have been reported. One example would be the H7N9 bird flu virus, which has killed 316 people globally.
The World Health Organization works rapidly to predict the antigen shape every month. They compare the different strains of flu virus to predict which are more likely to infect a lot of people, in the hope of delivering an effective vaccine. However, new technology, such as a universal flu vaccine, is undergoing clinical trials, in an attempt to permanently get rid of the flu.
Original image by Angel Mak