White blood cells fight infection

White blood cells are the main cells of the immune system and along with the tissue resident cells related to them they help fight infection. They are involved in defending the body against any infectious disease, pathogen or foreign material that may be dangerous to the body. Many kinds of leukocytes exist and all of them derive from one single hematopoietic stem cell.


Cells of the Immune System


Infection can be caused either by bacteria, virus or any other pathogen that may want to take advantage of the rich environment that our body provides. Leukocytes fight infection in a very complex way with multiple interactions among them, but can be explained with simple words. The body also has external protection that prevents pathogens from entering and causing infection: sweat, mucus, saliva, body secretions and little enzymes in our outer surface help kill these organisms before they can enter the body.

How does the body fight infection?

If the outer layer of protection is evaded (such as, a cut in the skin is made) organisms enter the body. An infection is made by pathogens and an inflammation response is the first thing that our body reacts with. Many things happen at the time in order to kill bacteria or virus and prevent the infection from spreading:

1. Macrophages act: these are large cells and are the general scavenger cells of the body. They phagocyte any dead cells and cell debris as well as invading microorganisms. Macrophages help fight the infection by eating up the pathogens.

2. Neutrophils: are effector cells of the innate (immediate) immunity that are rapidly mobilized to enter sites of infection. They are also phagocytes and along with macrophages, it is specialized in the capture, engulfment and killing of pathogens. Once macrophages have identified an infection, they call up the neutrophils to the site of infection through the release of special molecules called cytokines.

3. Complement System: One of the first weapons to fire is a system of little soluble proteins that are present in the blood, lymph and fluids. These proteins detect the presence of a bacteria or extracellular virus and coats it’s surface with the intention of “tagging” it as a pathogen in order for macrophages and neutrophils to take action.

4. Dendritic Cells: these are resident in the body’s tissues. Their main function is to detect pathogens, engulf them and leave the tissue with a cargo of intact and degraded pathogen. Dendritic cells (aka DC) act as a messenger goes to the lymph nodes were specialized B cells and T cells are and call the up to the site of infection.

5. B cells and T cells: These are more specialized immune cells and their action may take some days in order to be effective against infection. However, normal infections are controlled with the first step actions of the immune system. More chronic and severe infections may be a little more of a problem. B cells and T cells are specific to the pathogen, this mean, that they take some days in order to prepare themselves to attack only the pathogen that has been recognized in the infection site and not other microorganism. This specificity is good since the effectiveness of the eradication is impressive.

The first three steps (1, 2 and 3) happen at the time and these cells help each other. When infections become more serious DC come into play in order to get B cells and T cells into action. B cells and T cells form part of the adaptive immunity (a more specific response and takes longer to act), but are the only cells in the immune system that leaves in the body a “memory” in order to have a better response if out body gets a second contact with the pathogen. Vaccines are based in this idea of “immune memory.”

Immune system overview