04/01/2012 18:11 GMT | Updated 05/03/2012 05:12 GMT

Why Should You Care About Your Immune System?

It's easy to take your immune system for granted, yet every day billions of cells patrol your body and repel invaders. Just like a large corporation, each member of your internal army has a different role. Some act as scouts, recognising the presence of foreign proteins and sounding the alarm. Others act like bosses, super-stimulating the majority to do the dirty work of tackling fungi, bacteria and viruses. Once an attack is over, 'memory' cells remain behind like security guards to rapidly recognise and neutralise the same infection should you re-encounter it in the future.

While your immune cells remain vigilant all the time, they are less effective when you experience physical or emotional stress. How many times have you kept everything together through stressful times, only to come down with an horrendous cold as soon as you start to relax? How many holidays have been ruined by a cold sore, attack of thrush, cystitis or vomiting and diarrhoea? That's because stress hormones, such as cortisol, reduce the activity of white blood cells so they are less able to fight infections.

But even if stress is unavoidable, all is not lost - good nutrition can boost your immune reserves.

Fruit and vegetables are important sources of vitamins, minerals and phytochemicals designed to protect plants from bacteria, fungi and viruses but are just as effective in you. Select a rainbow of produce as those that are brightly coloured offer the highest immunoprotective content. Think green (peppers, spinach, broccoli), purple (red onions, cherries, beetroot, blueberries, black grapes), orange (apricots, carrots) and bright red (chillies, pomegranates, strawberries, tomatoes and red apples).

The old saying, 'An apple a day keeps the doctor away' is based on fact. People who eat an apple a day are one third less likely to succumb to any illness than people who don't eat apples. And grapes really are the ideal gift for convalescence - they contain a number of beneficial antioxidants that protect our genes from damage, as well as increasing the number of circulating immune cells.

Fish oils are accepted as good for the heart, brain and joints but did you know they also boost immunity? Omega-3 fatty acids help immune cells respond to incoming signals more quickly. Oily fish are also a source of vitamin D. A recent study involving over 19,000 adults and adolescents found that those with the lowest vitamin D levels were over a third more likely develop a common cold than those with the highest levels. As vitamin D status tends to fall in autumn and winter when sunlight is too 'weak' to stimulate vitamin D production in the skin, a supplement may be a good idea.

Combine your fish with onions and garlic and you'll keep more than vampires at bay. Both contain sulphurous chemicals with natural antiseptic, antibacterial and antiviral actions to reduce your risk of a cold. Garlic also contains ajoene, a substance that reduces inflammation and improves cold symptoms in a similar way to aspirin and ibuprofen.

Finally, fight 'like with like' using friendly, probiotic bacteria. These keep your immune cells on their metaphorical toes and stimulate the activity of T-lymphocytes - the 'manager' cells that regulate your immune responses. For example, the daily consumption of a fermented dairy drink, such as Actimel, increases the production of antibodies after influenza vaccination in older people. And in children, those taking a daily fermented drink had 19% fewer common infectious diseases over three months than those not taking probiotics. My three children have had a daily probiotic drink ever since they were weaned, and rarely succumb to the common infections going round.

Probiotics also inhibit the growth of harmful bacteria associated with gastroenteritis such as Salmonella, Shigella and Clostridium. They are especially helpful for people travelling abroad where hygiene practices are less rigid, and 'foreign' microbes you haven't encountered before are rife. When my husband and I travelled up the Amazon for two weeks, probiotics kept us safe from the gastroenteritis that plagued some in our group. We've also just returned from trekking the original Inca Trail, and while most of our companions experienced 'looseness' we escaped frequent acquaintance with the joys of the chemical toilet. Probiotics can also prevent the diarrhoea which can occur when you need a broad-spectrum antibiotic.

If your diet is not as good as it could be, you might even want to consider taking a multivitamin and mineral supplement. Lack of nutrients such as iron, zinc, selenium and vitamins A, C, D and E can each lower your immunity and increase your risk of infections.

So there you have it. Take care of your immunity with a good, healthy diet, and it will take care of you.