The immune system combats micro-organisms that have penetrated the body and removes the body’s cells if they are at the end of their lives or are no longer fulfilling their purpose.

The immune system protects us against a number of risks. It combats micro-organisms that have penetrated the body. In addition, it can destroy the body’s cells, if they are at the end of their lives or are no longer fulfilling their purpose (degenerated cells). We first recognise the importance of the immune defences when they no longer work correctly. People born with significant defects in the immune defences often die early from the causes that are frequently simple infections.

The immune system is a complex system of various organs, types of cells and molecules. In this section, you will learn how the immune system works, how the immune system responds to flu-like infections and how you can support your immune system.

How the immune system works

All living beings have systems that defend against the effects of the environment. The human immune system and that of most vertebrates is particularly complex and adaptable. Their immune responses consist of two different systems: the innate immune system and the adaptive immune system, which targets pathogens.

The innate immune response

The innate immune response serves as the first line of defence against pathogens. This is accomplished with the help of phagocytes and a system of plasma proteins called the complement system. Phagocytes, which are also called “eating cells”, include various types of white blood cells like monocytes, macrophages and neutrophile granulocytes. They engulf pathogens, cellular debris and foreign bodies inside the body in order to eat and destroy them (which is the origin of the term eating cells). In the larger framework, this process is expressed in the form of an infectious reaction.

Memory cells

During an immune reaction, so-called memory cells are formed from parts of the B and T lymphocytes. After surviving an infection, these memory cells can detect the foreign bodies even after many years and accelerate the immune response. This process is called “immunological memory”. The principle of vaccination relies on the fact that dead pathogens (or parts of them) can be introduced into the body in order to build an immunological memory against this germ.

Lymph nodes

The lymph node is responsible for the production and maturation of immune cells (immune organs). The primary immune organs include the thymus and bone marrow, where T and B lymphocytes are created. The spleen, the lymph nodes and the mucosa-associated lymphoid tissues (MALT) are some of the secondary immune organs. Lymphocytes interact with antigens in the tissue of these organs and collaborate with their associated cells. The MALT organs include the tonsils in the pharynx. Among other things, the immune system is controlled by plasma proteins circulating in the blood (the compliment system) and special messenger substances, the cytokines.

At birth, the specific immune system is not yet developed. Therefore, up to twelve colds per year are within the normal range for infants, and the course is generally uncomplicated.

In contrast, older people get sick more often and suffer more than the average population, because the immune system is also subject to the aging process. The production of lymphocytes becomes slower and the body regenerates poorly after infection as people age.

How the immune system responds

Even with an intact immune system, a cold cannot always be prevented. However, its function does affect the course of the cold. When the immune system is working better, the symptoms will be fewer and the period of illness will be shorter.

Cold viruses and other pathogens penetrate the body through the airway and make contact with the immune system there. Initially, pathogens make contact with the mucous membranes in the airway. These mucous membranes possess an efficient self-cleaning mechanism, mucociliary clearance. This mechanism works by producing mucous from the mucous membrane cells, which trap the foreign bodies. The mucous membrane’s fine hairs (cilia) transport the mucous and the trapped foreign bodies to the pharynx, where everything is swallowed and destroyed by stomach acid.

Mucociliary clearance does not work as well in the winter. Cold air can impair the function of the cilia and heated air causes the protective mucous to dry. Because of that, the cilia doesn't move as much and can stick to each other.

If cold viruses penetrate the mucous membrane in the airway, an innate defensive response will try to neutralize them. This will cause a localised infection response. In addition, the adaptive immune system will be triggered. Because however there are many different viruses that cause colds, a general immunity to colds does not exist.

With its typical symptoms, a cold breaks out when the virus reproduces so quickly that the body’s initial defensive responses do not suffice. The infection response becomes stronger and leads to a sore throat, blocked nose, a dry cough and sneezing. As the illness progresses, mucous production increases. The nose becomes blocked or runny, and the initial dry cough begins ejecting phlegm. The immune system will be able to handle everything again about a week after a typical cold has runs its course.

Even with an intact immune system, a cold cannot always be prevented. However, its function will affect the course of the cold and when the immune system is working better, the symptoms will be fewer and the period of illness will be shorter. In contrast, people with a weakened immune system often get one cold after another. Therefore, it is useful to provide additional support for the immune system.

Support the immune system

The best support for your immune system is a healthy lifestyle. This includes not too much stress, sufficient sleep, exercise, a balanced diet and sufficient fluid intake. The reasons for this vary.

Sleep

The proof that sleep is the decisive regeneration phase not just for the brain but also for the immune system has been increasing. Lack of sleep has a negative impact on the defensive cells of the innate and adaptive immune system. Sleeplessness due to emotional problems or especially stressful situations can have negative effects on the immune system.

Nutrition

The diet should be balanced and diverse. Fruit, vegetables and whole grain products should be included in such a diet. Daily intake of at least 1.5 litres (or better 2 litres) of fluid is recommended. A sufficient supply of Vitamin C and zinc is also especially important.

Hygiene

Aside from that, a couple of simple hygienic measure could protect against flu-like infections. Washing the hands is especially important in winter, because cold viruses can be transferred along with the droplet infections and through smear infections. Germs prefer to be on objects that are often touched, like door knobs.

Stress

There are also situations that lend themselves to increased infection rates. Too much stress is one such situation. As the situation becomes more stressful, the production of stress hormones, adrenaline, noradrenaline and cortisol, increases and the sugar and fat reserves are activated. This was reasonable in the Stone Age in order to be better able to fight or flee. Today, we no longer need this in stressful situations, but the associated responses remain and one of them is the downregulation of the immune system.

The immune system is particularly susceptible after a stressful situation. Infections, like a cold at the start of holidays, frequently arise once the stress has been eliminated. Therefore, it is a good idea to take it easy for 2-3 days after intense efforts. The same applies to athletes after intensive training. The window for increased susceptibility can remain open for several months during psychological stress like the illness or death of a family member. The reverse situation of a positive attitude towards life has a positive effect on the immune system.