The immune system – the body’s protective shield

It is present from birth and in many cases actually prevents pathogens from invading the body and facilitates a rapid immune response. Although it cannot adapt to new pathogens, its various responses provide an effective defense in many situations.

Impermeable interfaces provide an effective barrier

Mechanical barriers with different physiological defense mechanisms are the first line of defense. Our skin and mucous membranes – the respiratory tract, the eyes, the gut or the reproductive organs – form interfaces with the external environment. As they represent a potential portal of entry for pathogens, this is where the initial immune responses occur. That being said, intact cell organization already ensures good mechanical protection. This requires healthy cells and tight junctions between the cells.

Also, in play are biochemical substances such as sweat, saliva and mucus, which additionally contain antimicrobial proteins. Gastric acid also plays its part; thanks to its very low acidic pH it is able to kill numerous potential pathogens. The physical removal of pathogens or unwanted substances is facilitated either by cilia, for example in the respiratory tract, fluids such as tears, nasal secretions and urine, or bowel movements. The bacteria that colonize the human body also play an essential part in the immune defense – both the skin flora and especially the representatives of the intestinal flora (more on this below).

Multifaceted cellular defense

If pathogens – bacteria, parasites, fungi and viruses – overcome these barriers, white blood cells – leucocytes – come into play. These blood cells originate in the bone marrow and do not carry haemoglobin. Instead, they can

• differentiate between the surface structures of the body’s own cells and foreign cells,

• release various signal substances, thereby triggering inflammation,

• produce antibacterial proteins such as lysozyme or

• ingest and digest (phagocytosis) pathogens such as dead, defective or altered cells, e.g. virus-infected cells and tumor cells, from within the body – they are then called phagocytes.

White blood cells circulate in the blood, but they are capable of active movement and can therefore also migrate into tissue. Leucocytes include dendritic cells, granulocytes (basophils, eosinophils, neutrophils), macrophages and their precursor cells, the monocytes, but also mast cells and natural killer cells. Killer cells bring about the cell death of altered cells from within the body, such as virus-infected cells and tumor cells.

Another element of the non-specific immune system is the complement system, which consists of specific proteins that are activated via a cascade. Their main role is to tag pathogens so that it is easier for immune cells to identify and attack them. In addition, these proteins can trigger inflammation and destroy bacterial envelopes.

Inflammation can also be beneficial

Inflammation is an immune response to harmful stimuli induced by signal substances (e.g. cytokines). It should result in more rapid healing, by attracting immune cells that remove or destroy foreign substances and pathogens.
However, the body’s own tissues can be damaged in the process, which is why severe and especially also recurrent inflammation is problematic. If inflammation is directed against harmless environmental stimuli such as pollen, experts call this an allergy, which is generally caused by the adaptive immune system. When the inflammatory response targets the body’s own structures and cells, this is known as autoimmune disease.

The adaptive immune system develops throughout our life and is fully formed from puberty. The key fact: it can adapt to new or altered pathogens. Protein, lipid and carbohydrate compounds or complex molecules on the cell surface act as antigens. White blood cells in the form of B and T lymphocytes (B and T cells) carry receptors on their surface that can only bind to specific antigens, like a lock-and-key system.

Various cells, such as dendritic cells, present antigens from pathogens on their cell surface following phagocytosis of the pathogens. B cells with corresponding receptors respond to the relevant antigens by transforming into plasma cells and producing corresponding antibodies. The antibodies bind to the respective antigens, thereby

• blocking the antigens and inhibiting their harmful effect;

• tagging pathogens for phagocytes;

• activating the complement system, whose components destroy the tagged cell;

• tagging altered endogenous cells for natural killer cells.

T cells recognize the body’s own cell surface structures and respond to altered or foreign structures on body cells by differentiating into so-called effector cells: as killer T cells they destroy altered cells, as T helper cells they attract other immune cells or activate B lymphocytes, and as regulatory T cells they protect intact body cells. Regulatory T cells thereby direct the immune response, aid immune tolerance and prevent autoimmune diseases.

As it can take around four to seven days until the adaptive immune system is able to fight foreign pathogens that it has encountered for the first time, the innate immune defense system is required initially. However, both B and T lymphocytes form long-lived memory cells which produce antibodies and effector T cells much more rapidly when they come into contact with the same pathogen again. In the best case, they ensure lifelong immunity to the relevant pathogen. Vaccines also make use of the principle of memory formation.