Article Series 1: The Blood and the Immune System

The Components of the Blood and the Immune System

We all have a general idea of what blood is. The heart pumps it through our body, and we need it to live. It is used to swear eternal friendship. We use it to save lives. If we are sick, the doctor can test our blood to find out what is wrong. But how can the blood reflect how we are sick, and why is it so essential to life? The biology of the blood is fascinating, and in the following educational article series, I will introduce what blood really consists of, and discuss the function, the development, and some of the diseases of our blood cells and our immune system.

 

What are the components of blood?

Your blood makes up about 7% of your body weight, and in an adult amounts to around 5 liters (= 1.3 gallons)¹. If you lose too much blood, for instance during a trauma or an accident, it can be life-threatening, and you may need a blood transfusion. We are able to donate up to half a liter, which is about 10% of our total blood volume, however such blood donation requires overall good health, and time to rest and recover afterwards. Roughly half of the blood volume consists of various blood cells, while the other half is blood plasma, the liquid which enables your blood to flow throughout your body. Every component in the blood has their own critical function, which will be introduced in this article.

 

The blood plasma consists of approximately 90 % water¹

Water is critical for life. A human can survive for days and even weeks without food, but only a few days without water. The reason is that water is a resource that constantly recycles. We lose water from our body both through urine and as evaporation from our skin through sweat. Neither of these processes are something we can consciously control, but they are important processes for temperature control as well as getting rid of waste products. On the other hand, our water intake is under our control. We get water from what we drink, but also through food. Mild dehydration may lead to headache, overheating, or dizziness, but is not life-threatening under normal circumstances. In cases of extreme dehydration, you can get liquid  through intravenous transfusions directly into your blood.

 

The blood plasma contains various soluble components

In addition to giving blood its fluidity, so that blood cells can be transported throughout the body, water is also important as a solvent for transport of nutrients and waste products. Minerals, vitamins, glucose, and various types of proteins, along with the water, make up the blood plasma. Although the color of the blood is red, the color of the blood plasma is actually yellow. The red color comes from the large amount of red bloods cells, as will be described below. The yellow color of the blood plasma comes from the various water-soluble components, such as nutrients and various signaling molecules. Furthermore, your blood is the carrier of various waste products, that are filtered out from the blood to the urine through the kidneys. In addition, the blood contains various proteins that have both structural as well as regulating or signaling roles. One type of important structural proteins are the coagulation factors that are required for proper blood clotting. Insulin is an example of signaling molecule. People suffering from diabetes must closely monitor and adjust the glucose and insulin levels in their blood, to ensure a proper balance.

 

The cells in our blood

The cells in our blood are divided into two main types: The red blood cells, and the white blood cells. In addition, there are specialized cell fragments, called platelets, that are derived from a specific type of white blood cells, the megakaryocytes. The red blood cells (RBC, also called erythrocytes) take up about 45% of the total blood volume¹. The red color is due to abundant amounts of the protein hemoglobin, which binds and transports oxygen from the lungs throughout our body. The white blood cells are critical for our immune system, which can broadly be divided into the innate and the adaptive immune system. The innate immune system recognizes patterns that are associated with pathogens, and mounts a fast reaction towards infections. The adaptive immune cells recognize specific eptiopes, and can be educated to recognize epitopes associated with disease. The response of adaptive immune cells are initially slower, but the education leads to a "memory" so that upon later encounters, we can quickly recognize and eliminate the threat. Immunization is based on the ability of the adaptive immune system to recognize the pathogen and develop a protective "memory" or immunization. Lastly, the platelets, also called thrombocytes, are not cells, but rather cell fragments. They are critical for blood coagulation, to ensure that upon a cut or damage to a blood vessel, the bleeding will stop.

 

Summary

Human blood has many well defined components, and is the major transport system of both nutrients, signaling molecules, waste products and immune cells. Therefore, a simple blood sample can reveal many types of disease or unbalance in the body, and thereby help with initial diagnosis.

 

Reference:

Molecular Biology of the Cell, Fourth Edition, 2002, Editiors: Alberts et al., Publisher: Garland Science

Glossary:

Adaptive Immune System: Is the part of the immune system that can adapt to and "remember" the pathogen based on "memory". This feature is unique to higher vertebrates, and is based on a clonal expansion of cells that are able to react to a pathogen. Immunizations take advantage of the adaptive immune system, by introducing the body to potential pathogens (for instance the inactivated flu virus) to elicit a memory response to protect you from future exposure.

Epitope: A surface that can be recognized by the immune system. It is also called an antigenic determinant, as it is a part of an antigen, the unit that an antibody or immune receptor recognize. The epitope can be a linear sequence of for instance amino acids (a small part of a protein, called a peptide), or it can be a 3-dimensional surface composed of different parts. 

Innate Immune System: Is the part of the immune system that is innate, that we are born with. It generally recognizes so-called Pathogen-Associated Molecular Patterns (PAMPs), for instance common repetitive structures on the surface of bacteria and viruses.

Pathogen: A foreign organism that leads to damage or disease (pathology). Bacteria, virus, parasites are all examples of potential pathogens. However, the commensal bacteria in your gut, or the cultures in yoghurt, are not pathogens. 

Acknowledgement: Thanks to writer/director Jennifer Sharp for critical review of and valuable feedback for this article.

 

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