Our immune system is the first line of defence against illness (Photo: Shutterstock) Our immune system is the first line of defence against illness - everything from colds to Covid. The coronavirus outbreak has pushed our immune system centre-stage and made us realise the importance of having healthy defences. The core of the immune system is a vast army of white blood cells, otherwise known as immune cells, which fend off against viruses, fungi and parasites that make us ill. There are two types of immune cells, the front line defenders that are all about speed and aggression and the specialists that come in to produce custom made attacks over a longer period.