White Blood Corpuscles (WBCs) are cells in the immune system that protect the body against infectious diseases and foreign invaders. WBCs, aka Leukocytes, are produced from multi-potent cells in the bone marrow of the human body called haematopoietic stem cells. They are found all over the body, in the blood and lymphatic system as well.
WBCs can be classified by structure or by cell lineage. When classified by structure, there are Granulocytes and Agranulocytes; by cell lineage, there are Myeloid Cells, and Lymphoid Cells. These categories are further divided into five main types, namely, Neutrophils, Eosinophils, Basophils, Monocytes, and Lymphocytes. Further subtypes can be classified; for example, lymphocytes have three kinds, i.e., B cells, T cells, and NK cells.
The two basis of differentiation are easily distinguishable. Granulocytes have a lobed nucleus shape (polymorph-nuclear) and contain granules in their cytoplasm (visible under light microscopy); agranulocytes have a round nucleus shape (mono-nuclear) and do not contain granules in their cytoplasm. The other dichotomy offers myeloid cells and lymphoid cells; myeloid cells are distinguished by their haematopoietic lineage. Myeloid cells are classified as Neutrophils, Eosinophils, Basophils, and Monocytes, while lymphocytes can be further classified as T Cells, B Cells, and Natural Killer Cells.
Neutrophils are the most abundant WBCs that defend against bacterial or fungal infection. They are usually first responders to microbial infection causing their activity and death in large numbers to form pus. They are active in phagocytosing bacteria and are present in large amount in the pus of wounds. They are abundantly found during the early stages of acute inflammation. The life span of a circulating human neutrophil is about 5.4 days.
Eosinophils compose about 4% of the WBC total count, which fluctuates throughout the day, seasonally, and during menstruation, that rise as a response to allergies, parasitic infections, collagen diseases, and disease in the spleen and CNS. Eosinophils mostly deal with parasitic infections and are the predominant inflammatory cells in allergic reactions like asthma, hay fever, hives, and parasitic infections.
Basophils are responsible for allergic and antigen response through the release of histamine that causes dilation in blood vessels; this can be identified by several coarse, dark violet granules, giving them a blue hue. They excrete histamines and heparin to aid in the body’s defenses; histamines increase the flow of blood to the injured tissue and heparin promotes the movement of WBCs into the area. Basophils release chemical signals that attract eosinophils and neutrophils to an infection site.
Monocytes are the largest type of WBCs that are used for phagocytic functions and offer pathogens particles to T cells so that pathogens once recognized, are killed. Soon, they leave the bloodstream and become tissue macrophages that remove dead cell debris and attack microorganisms. Monocytes, unlike neutrophils, are able to replace their lysosomal contents and are thought to have a much longer active life.
Lymphocytes are common in the lymphatic system and can be differentiated by having a deep staining nucleus that may be eccentric in location, and a considerably small amount of cytoplasm. B Cells create antibodies that bind to pathogens, block pathogen invasion, activate the complement system, and enhance pathogen destruction. T Cells are helper cells and killer cells; helper cells (CD4+ helper T Cells) arrange an immune response an play important roles in all arms of immunity while killer cells (CD8+ cytotoxic T Cells, γδ T Cells) hunt down an destroy cells that are infectious or cancerous. Natural Killer Cells provide rapid responses to virus-infected cells and respond to tumor formation.
One indication of the presence of a disease is the number of leukocytes in the blood; thus, the WBC count is an important subset of the complete blood count. The normal WBC count is between 4 × 109/L and 1.1 × 1010/L. Leukocytosis is found when there is an upward spike in the number of leukocytes over the upper limits. It is normal when it is part of healthy immune response; it is occasionally abnormal, when it is neoplastic or autoimmune in origin. A decrease below the lower limit is called Leukopenia; it indicates a weakened immune system.