Lymphocytes are complex cells that direct the body’s immune system. T lymphocytes start in the bone marrow from pluripotent hematopoietic stem cells, then travel to and mature in the thymus gland. The thymus is located in the chest between the heart and sternum (breastbone). B lymphocytes mature in the bone marrow.

T lymphocytes (T cells) are responsible for cell-mediated immunity. B lymphocytes are responsible for humoral immunity (antibody production). Seventy-five percent of lymphocytes are T cells. Lymphocytes are different from the other WBCs because they can recognize and have a memory of invading bacteria and viruses. Lymphocytes continually pass back and forth between lymph tissue, lymph fluid and blood. When they are present in the blood, they stay for several hours. Lymphocytes can live for weeks, months or years.

Platelets (thrombocytes) help blood to clot by forming something called a platelet plug. There are approximately 150,000 to 400,000 platelets in each microliter of blood (average is 250,000). Platelets are formed in the bone marrow from very large cells called megakaryocytes, which break up into fragments - these cellular fragments are platelets. They do not have a nucleus and do not reproduce. Instead, megakaryocytes produce more platelets when necessary. Platelets generally last for an average of 10 days.