White blood cells definition, in broad sense, can be said to be a collection of colorless cells that make up the immune system. They either circulate in the lymphatic system or in the blood. They are solely responsible for the reactions that occur in the body when foreign micro-organisms find their way in. What are white blood cells?

White Blood Cells Definition

The immune system contains cells that are charged with protecting the human body against foreign invasion and diseases. These cells are referred to as white blood cells, or called leucocytes or leukocytes. Hematopoietic stem cell is a multipotent cell found in the bone marrow and it is responsible for the production of leucocytes. Leucocytes are found all over the body, including the lymphatic system and the blood.

The count of white blood cells in your blood is in most cases can be an indicator of an infection. Normally, the white blood cell count should be 4 to 11 x 109/L. In America, this is expressed as that per microliter of blood contains 4000-11000 white blood cells. In a healthy adult's body, white blood cells make up 1% of the blood volume. When the leucocyte or white blood cell count is above the upper limit, it is referred to leukocytosis and below the lower limit is referred to leukopenia.

Five Types of White Blood Cells

Since you have known the white blood cells definition, you should also be clear that there are 5 different types of white blood cells. The diverse functionalities and physical characteristics are what differentiate them.

1. Neutrophil

Neutrophil is about 10-12 μm in diameter with a fine, faintly pink color. They constitute 60%-70% of the leukocytes in circulation, making them the most abundant type of white blood cells. Neutrophils protect the body against fungal and bacterial infections. They respond first to any microbial infections, and pus forms as a result of their death and activity. They are commonly referred to as PMN (polymorphonuclear) leucocytes. Their nucleus has 3-5 lobes joined by thin strands. For this reason, they appear to have multiple nuclei, thus given the name polymorphonuclear. However, PMN refers to all granulocytes in technical sense. Their cytoplasm is clear and they are not capable of renewing themselves, so after phagocytosis of a few pathogens, they die.

2. Eosinophil

Eosinophil is about 10-12 μm in diameter with a full of pink-orange color. They form only 2%-4% of the total white blood cells. However, this number often changes during the day and during menstruation. Besides, when there are allergic reactions, collagen diseases, parasitic infections, disease of the spleen and central nervous systems, they increase in number. They are hardly found in the blood, but are rich in the mucous membranes of the digestive, respiratory and urinary tracts. They basically combat parasitic infections, but they also deal with allergic reactions like asthma. Eosinophil produce chemicals that terminate large parasites like tapeworms and hookworms that cannot be phagocytized. Eosinophil has a bi-lobed nucleus connected by a slender strand.

3. Basophil

Basophils' main function is to respond to antigen and allergic reactions. They are the least of the white blood cells and constitute about 0.5% of their total count. They are coarse and have dark violet granules, which give them a blue shade. They have a tri or bi-lobed nucleus. They produce heparin and histamine. Histamine dilates the vessels increasing blood flow to the injured area. It also increases the permeability of the vessels to allow clotting proteins and neutrophils to move easily. Heparin, on the other hand, prevents clotting since it is an anticoagulant and eases the movement of white blood cells to a specific location. Basophils also release chemical signals that call out neutrophils and eosinophil to an infected area.

4. Lymphocyte

Lymphocytes are mostly found in the lymphatic system and not in the blood. Their cytoplasm is relatively small with a deeply stained nucleus located centrally. Lymphocytes include:

• B cells that produce antibodies which bind, block and destroy pathogens. They also activate the complement system.
• T cells: there are 3 types off T cells.

–   CD4+ Th (T helper) cells that bind antigen peptides and the helper T cells coordinate immunity and make cytokines.

–   CD8+ cytotoxic T cells bind antigens in MHC I complex of tumor or virus-infected cells and destroy them.

–   Natural killer cells destroy cells that do not show MHC class I molecules or those that have stress markers like MHC class I polypeptide-related sequence A.

–   γδ T cells are more common in tissues than in the blood. They have similar characteristics to T helper cells, natural killer cells and cytotoxic T cells.

5. Monocyte

Monocytes share the same function as the neutrophils but do not die easily. They are also tasked with an extra role of presenting parts of pathogens to T cells so that these organisms can be identified again and destroyed. They contain a kidney shaped nucleus, and with time they leave the blood to become tissue microphages that move dead cells and combat microorganisms.

White Blood Cells Related Problems

If you want a thorough understanding of the white blood cell definition, you should know the following information. For example, if your white blood cell count is low, it means that the cells are dying faster than the body is replacing them, which can make you vulnerable to fatal diseases. Likewise, if the count is too high, then it is an indication of a problem. Here are some other common problems related to white blood cells.

• Weakened immune system: This is as a result of HIV/AIDS or cancer treatment that destroys the white blood cells.
• Infection: When the white blood cell count is high means that your body is fighting an infection.
• Myelodysplastic syndrome causes abnormal cell production in the bone marrow.
• Cancer of the blood, both leukemia and lymphoma, causes growth of abnormal blood cells, increasing risk of bleeding and infections.
• Myeloproliferative disorder is caused by conditions that cause excessive production of immature cells. This, in turn, leads to a very high or low count of white blood cells.