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Lymphocytes and monocytes are both white blood cells; together, they make up the majority of the function of the lymphatic immune system. They are similar in that they are both classified as mononuclear cells, meaning that their nucleus is formed in one piece, but differ in their function within the immune system. Lymphocytes are responsible for the body’s immune response when a recognized foreign invader attacks healthy tissue because they have the capability to make specific antibodies against a pathogen. Monocytes look similar to lymphocytes in many instances, but when a monocyte performs its unique function as a macrophage, engulfing and digesting invaders that the lymphocytes identify, they become significantly larger and irregular in shape. Lymphocytes and monocytes also exhibit other distinctive characteristics that help to distinguish them under the microscope, like when monocytes form “feet” that help them propel quickly through the lymphatic and circulatory system.
These white blood cells differ in general number within the body as well. In a healthy individual, there are many more lymphocytes than monocytes. Lymphocytes make up to 20 to 40 percent of a healthy adult’s total white blood cell count and monocytes only comprise 1 to 6 percent of total blood volume. This fact can help observers to read a blood sample more quickly as most mononuclear cells can be identified as lymphocytes if they do not readily exhibit characteristics of a macrophage. Young children can be expected to have an even higher ratio of lymphocytes to monocytes because their bodies are continually trying to build immunity to new infectious agents acquired in the environment.
Under observation by a microscope, the smallest, most easily distinguishable lymphocytes are almost perfectly round and have bluish cytoplasm. When lymphocytes encounter an invader with which they are familiar, they can grow in size, but usually remain similar in composition. In some cases, however, lymphocytes can exhibit uncharacteristic behaviors, like stretching and becoming wavy, a case where they may be mistaken for a different cell type. In comparison, monocytes are observed as having irregular, incongruous shapes and sometimes appear granulated if they form vacuoles or other apparatus used in common macrophagic activities. The nucleus of macrophagic monocytes may look “brain-like” because it tends to fold in on itself and form convolutions.
The most significant difference between lymphocytes and monocytes can be comprehended by studying their life cycles. Interleukin-7 (IL-7) is responsible for the white blood cells leaving the bone marrow where they are formed. They begin down the “lymphoid” path, where further differentiation between lymphocytes and monocytes takes place. For example, a chemical, macrophage colony-stimulating factor (M-CSF) helps some mononuclear cells become macrophages. Lymphocytes are stimulated by other factors and transform into the immune system’s main defense mechanisms, B cells and T cells.