Why are babies more susceptible to meningitis and septicaemia?

Professor Nigel Klein answers:

We are surrounded by thousands of different types of bacteria, viruses, fungi and parasites. However only a few have the capacity to cause infection in humans. To cause an infection, they have to get past the body’s outer defences, such as the skin, and then find a place within the body where they can grow and spread without being destroyed by the immune system.

The skin is very effective at preventing infections at all ages. However as we have to eat, drink and breathe, we cannot be completely covered in skin, and so we are vulnerable to infection at sites such as the throat, ears, chest, bladder and gut. Infections at these sites with the bacteria that can cause meningitis and septicaemia are more common in young children because the immune system is not yet fully developed.

After birth, babies have protection against infection through two main parts of the immune system. Firstly, babies are protected by what is known as the innate immune system. The innate immune system provides an immediate defence against disease causing bugs and is very effective. However, it doesn’t protect against all bugs, isn’t at its best early in life and doesn’t learn, so protection doesn’t get better with time.

The second tier of protection in babies comes from antibodies called immunoglobulins. They are passed from mothers in the last 12 weeks of pregnancy. They are very important and provide good protection until 3- 6 months of age when their levels decrease. This decline, coming as it does before the baby’s own defences have developed, leave babies particularly susceptible to infections.

These antibodies belong to the adaptive immune system. Unlike the innate immune system, with each infection the adaptive immune system develops highly specific protection against a multitude of bugs. It does this by memorising each bug as it encounters it. But this takes time and while this learning system is developing, children, particularly between 6 months and 2 year, are more likely to get infections.

Vaccines can speed up this learning process. With vaccines we can train the immune system to fight infection. Even though the immune system is not mature in babies and young children, by giving enough doses (often three), the immune system can learn to fight many infections. However, not all vaccines work well in young children, so there is considerable research to try and develop ones that work well in children of all ages.

Our expert – Professor Nigel Klein 

Professor Nigel Klein

Nigel Klein is Professor and Consultant in Paediatric Infectious Diseases and Immunology at Great Ormond Street Children’s Hospital, London, and the Institute of Child Health, University College London. He trained at UCL, obtaining degrees in Anatomy and in Medicine. He worked in the three London centres specialising in Paediatric Infectious Diseases before completing his formal training at ICH/GOSH. He is currently Head of Infectious Diseases Unit at ICH and was Head of the Department of Infection at UCL until 2008. He has been working in the fields of meningitis and sepsis for many years, in both a clinical and academic capacity. Nigel is a scientific advisor for a number of medical research charities including MRF.  

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