Lymphocytes are a type of white blood cell that play a crucial role in the immune system of most vertebrates. They are formed in the bone marrow and are dispersed throughout the body, but once an organism or virus invades, they immediately gather to fight it off. There are two main types of lymphocytes: T cells and B cells.

• T cells: T cells control the bodys immune system response and directly attack and kill infected cells and tumor cells.
• B cells: B cells make antibodies, which are proteins that target viruses, bacteria, and other foreign invaders.

Lymphocytes help the immune system fight cancer and foreign viruses and bacteria (antigens) . They also help the immune system remember every antigen it comes in contact with. After an encounter, some lymphocytes turn into memory cells. When memory cells run into an antigen again, they recognize it and quickly respond. This is why people don’t get infections like measles or chickenpox more than once. It’s also the reason getting vaccinated can prevent certain diseases.

A lymphocyte count is usually part of a peripheral complete blood cell count and is expressed as the percentage of lymphocytes to the total number of white blood cells counted. A general increase in the number of lymphocytes is known as lymphocytosis, whereas a decrease is known as lymphocytopenia.