White blood cells, also known as leukocytes, have an important role in the immune system, as well inflammatory responses. Leukocytes act to defend the body from bacteria, viruses and parasites, and also help to protect against diseases such as cancer. Understanding the functions and relative numbers of the different types of white blood cells in circulation can provide insights into disease diagnosis and management.
Neutrophils
Neutrophils make up the majority of white blood cells in the circulation. The Lymphoma Information Network explains that neutrophils are the body's first line of defense against bacterial and viral infections and certain diseases. Neutrophils have an important role in inflammation resulting from cuts or infection. Certain diseases and toxins can lower neutrophil count, leading to a condition called neutropenia. Individuals with neutropenia are at an elevated risk for infection and disease.
Lymphocytes
Lymphocytes are the second most numerous white blood cell type in the body. The University of Nebraska describes lymphocytes as large cells with clear cytoplasm and a relatively large nucleus. Lymphocytes have an important role in defending the body against disease. Certain lymphocytes known as T-lymphocytes fight virally infected cells, as well as tumor cells. B-lymphocytes are important as they produce antibodies. High lymphocyte counts may be indicative of a chronic viral infection. Low T-lymphocyte counts can serve as an indicator of HIV or AIDS progression.
Monocytes
Monocytes are the largest type of white blood cell in circulation. These white blood cells are involved in defending the body from bacteria and viruses. The University of Nebraska explains that monocytes can leave the blood stream and develop into macrophages, a type of cell that eats bacteria and viruses through the process of phagocytosis. Certain infections such as malaria and Rocky Mountain fever can increase monocyte counts.
Eosinophils
Eosinophils make up about seven percent of circulating white blood cells. Though relatively low in number, these cells have an important role in defending the body from parasites. Eosinophils also play a part in allergic inflammatory responses. According to the Merck Manual, high levels of eosinophils, a condition known as eosinophilia, may be the result of infection or an allergic reaction. Low levels of eosinophils can occur in response to stress, treatment with corticosteroids or Cushing's syndrome.
Basophils
The University of Nebraska explains that basophils represent the least common type of white blood cell. Nonetheless, basophils have important roles in immune response. Basophils can be identified by the presence of large granules that contain histamine. According to the National Institutes of Health MedlinePlus, activated basophils release histamine that acts on blood vessels to induce vasodilation during immune response. An abnormally high basophil count is rare, but may occur during chickenpox.
