Lymphocytes are much more common in the lymphatic system, and include the so-called " killer T-cells ". The blood has two basic types of lymphocytes: B cells and T cells.

T-cells (capture above) recognize the invader cells. Different types of T-cell have different functions. Some send chemical instructions (cytokines) to the rest of the immune system. The other white cells then produce the most effective weapons against the invaders, which may be bacteria, viruses or parasites. Other types of T-cells (natural killer cells) recognize and kill virus-infected or cancerous cells directly. Some help B-cells to make antibodies, which circulate and bind to antigens.

T-cell killing cancer cell

B-cells: With the help of T-cells, B-cells make special Y-shaped proteins called antibodies. Antibodies stick to antigens on the surface of germs, stopping them in their tracks, creating clumps that alert your body to the presence of intruders. Your body then starts to make toxic substances to fight them. Patrolling defender cells called phagocytes engulf and destroy antibody-covered intruders.