The National Cancer Institute describes a leukocyte as a type of cell belonging to the immune system. When microbes such as bacteria or viruses infect the body, leukocytes respond to attack the microbes and clear the infection. Leukocytes are also sometimes called white blood cells, or WBC, which differentiates them from red blood cells, or RBC, which have hemoglobin and carry oxygen to the body's tissues. There are three main types of leukocytes, which have distinctly different mechanisms of action.
Lymphocytes
Lymphocytes are a very important type of leukocyte that specifically attacks infectious agents. Lymphocytes are part of the "adaptive immune system," meaning each cell evolves to specifically recognize and attack microbes that infect the body. According to Dr. Mike Farbee, PhD from Estrella Mountain Community College, T cells are a subtype of leukocyte that can directly kill viruses.
They can also help B cells, another subtype of leukocyte, secrete antibodies. Antibodies are a type of protein that are released from the cell and specifically stick only to infectious agents, marking them for destruction. According to Dr. David Wiseman, PhD, a professor at Central Virginia Community College, lymphocytes make up about 25% of all leukocytes.
Macrophages
Dr. Farbee describes macrophages as another type of leukocyte. Macrophages release substances that help other leukocytes, such as lymphocytes, grow. Macrophages can also engulf bacteria and kill them.
One way macrophages identify bacteria as a pathogen is by detecting the above-mentioned antibodies, which are secreted by B cells. Once the macrophages "eat" invading microbes, they process the microbe and display parts of the microbe on the macrophage cell's surface. T cells then interact with the macrophage, and they note the parts of the microbe on the macrophage cell's surface.
The T cells then know an infectious agent is present and what type of microbe they need to find. Wiseman adds that macrophages, which grow from precursor monocytes, make up about 6 percent of all leukocytes.
Granuloctyes
Dr. Wiseman states that granuloctyes, also called granular leukocytes, are another type of leukocyte. These cells get their name from their appearance under the microscope: the inside of the cell has a "granular" appearance, meaning there appears to be distinct solid granules inside the cell. In contrast, lymphocytes and macrophages are sometimes grouped together under the umbrella term "agranulocytes," meaning that they do not contain any granules.
There are three main subtypes of granulocytes: neutrophils, which comprise 65% of all leukocytes; eosinophils, which comprise 3% of total leukocytes; and basophils, which make up 1% of all leukocytes. These granuloctyes subtypes differ based on the granules inside of the cell. In response to infection, or exposure to an allergic substance in the case of eosiniphils, granulocytes release their internal granules.
