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Immune System: how does it work against an infection ?

A short introduction to the function of our immune system

What is a pathogen ?

Bacteria are organisms constituted of one cell (size in µm: one millionth of a meter) and are able to grow and reproduce. Viruses are smaller (size in nm: one billionth of a meter) and are constituted of an envelope with some proteins and genes inside. To fulfil their objective to reproduce, they need to invade a cell and hijack its mechanisms. Many microorganisms live with us in harmony (commensals), but some of them can damage our tissues, which make them pathogens.

What is the immune system ?

The immune system is composed of many different cells (white blood cells = leukocytes), mechanisms and molecules, which help our body fight against any foreign body, such as viruses, fungus or bacteria. Our immune system also fights against other physical aggressions to our cells (shocks, toxin, acids, dust…).

First : cutaneous barrier

Our first protection is physical, in particular the skin, the mucosa and the digestive tract. The presence of mucous, healthy bacterial flora, some chemical molecules like enzymes and anti-microbial peptides help to defend us too. Some of our organs can also expel pathogens : lungs through coughing and sneezing, urinary tract, eyes through tears…

Second : innate immunity

Then, we have immune cells scattered through our body, which guard our tissues. So, when a pathogen succeeds to enter our body and begins to multiply, our immune cells will often rapidly detect it and fight it. This first line of defence is called innate immunity. Cells from innate immunity (macrophages, neutrophils, NK cells, dendritic cells…) will begin an immediate and non-specific fight by secreting damaging molecules and engulfing (phagocyting) pathogens. The cells already on-site will attract more cells by releasing chemical mediators (cytokines, chemokines…) to inform the body about the site of the infection. This will induce inflammation, which symptoms are redness, heat, pain and swelling, caused by an increased blood flow and an infiltration of immune cells in the attacked tissue. Note that inflammation is a good thing at this point because it helps us fight the pathogen and activates the innate immune system (only chronic inflammation, often due to our lifestyle or to chronic disease, is a problem, because our body is constantly fighting).

Sometimes with fever : why ?

Our body can increase its temperature to try to neutralize pathogens, which are often heat sensitive. Thus, a mild fever is usually not a problem and can even be beneficial. The best thing to do is to rest and stay hydrated. Nevertheless, a temperature higher than 38°C needs to be monitored and you should contact your doctor, to find the cause of the infection and eventually take antipyretics.

Third : adaptative immunity

Rapidly, some of the cells* present on-site will englobe and “eat” the pathogen (phagocytosis), degrading it into small pieces. They will then migrate towards the closest lymph node or the spleen, to inform the rest of the immune system about the type of infection occurring. The aim is to create a response which is specific to the invading pathogen, called adaptative immunity. The lymphocytes are mainly the cells taking care of this kind of immune reaction. We have two types of lymphocytes: T cells, which are especially good at dealing with problems inside the cells (for example cells infected by a virus or cancer cells) and B cells, which generally take care of problems in our fluids (often bacteria).  We have a high number of lymphocytes in our body and each one of them is a little different from another, which enables our body to react against all kinds of pathogens. All of them circulate in our lymphatic system, and thus pass through the lymph nodes. When our phagocytic cells* arrive in the lymph node, they will present the little pieces of the destroyed pathogen and test every lymphocyte to find the exact one which can react against it. Once the appropriate lymphocyte has been found, it will be multiplied and activated. If the pathogen is a bacteria, it will be a role for B cells (humoral immunity), which will multiply and mature (becoming plasma cells) and then be able to secrete antibodies. These antibodies will stick to the pathogens, immobilise them, and allow other killing cells like macrophage to target the invading pathogen. If the pathogen is a virus, selected T cells will go to an organ called the thymus to multiply and be activated (cell-mediated immunity). Some of these T-cells will then be able to bind the infected cells and release molecules to kill them, other T-cells will stimulate other immune cells to improve their response and some will regulate them so as to avoid damaging the tissues.

It is important to know that this specific adaptative immune response takes almost a week to be ready.

After the infection: an enhanced protection next time, thanks to memory lymphocytes

Once the infection is defeated, everything comes back to normal, but the body will still keep a memory of this infection, through memory lymphocytes. Thus, if the same pathogen infects us again, our body can react much faster and more efficiently. Vaccination uses this principle to train our body to fight against a pathogen and be able to kill it fast in case a real infection begins.

The immune response can be hijacked

Unfortunately, some pathogens evolve and are able to avoid detection by our immune system, neutralize our immune response or even use it to their own advantage.

What to do when an infection occurs ?

The body needs plenty of rest and to stay well hydrated. Contact your doctor for a precise diagnosis and treatment. Be aware that antibiotics work against some bacteria, but never against viruses, so avoid self-medication. 


Please note that there are many other cells that can be implicated in our immune response and that the interactions between the different mechanisms are infinitely more complex. This document is just meant to highlight the main phases of the immune response to an infection.

This article is intended to summarize the basics of how a part of the human body works, but in no way replaces medical diagnosis and treatment.

Disclaimer of liability:
The information published on does not claim to be complete and is not a substitute for individual medical advice or treatment. It cannot be used as an independent diagnosis or to select, apply, modify or discontinue treatment of a disease. In case of health problems, it is recommended to consult a doctor. Access to and its contents is at the user’s own risk.


  1. Aeberli, I. et al. Low to moderate sugar-sweetened beverage consumption impairs glucose and lipid metabolism and promotes inflammation in healthy young men: A randomized controlled trial. American Journal of Clinical Nutrition 94, 479–485 (2011).
  2. Joseph, S. v., Edirisinghe, I. & Burton-Freeman, B. M. Fruit Polyphenols: A Review of Anti-inflammatory Effects in Humans. Critical Reviews in Food Science and Nutrition 56, 419–444 (2016).


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