Monocytes take in (or ingest) foreign particles. Once monocytes migrate from the bloodstream and enter tissue they are known as macrophages. Macrophages also play a key part in alerting the rest of the immune system of invaders.

Granulocytes (also known as myelocytes) contain granules filled with potent chemicals that allow them to destroy the enemy.

There are two types of lymphocytes - B lymphocytes and T lymphocytes. B lymphocytes, or B cells, are like the body's
military intelligence system, seeking out their targets and sending defenses to lock onto them. T lymphocytes, or T cells are like the soldiers, destroying the invaders that the intelligence system has identified. T cells are crucial to the immune response because they possess a unique 'memory' system which allows them to remember past invaders and prevent disease when a similar invader is encountered again. There are three types of T cells - "helper" T cells (CD4+), "killer" T cells (CD8+) and suppressor.

Natural killer cells kill on contact. The killer binds to its target, aims its weapons, and then delivers a lethal burst of chemicals that produces holes in the target cell's membrane.

Dendritic cells usual function is to identify a foreign substance, including cancer cells, and process the bits of such foreign substances and then jumpstart the immune response by bringing the foreign substance to the attention of the rest of the immune system (mostly T lymphocyte cells). Given their unique antigen presenting properties, dendritic cells are currently being developed for immunotherapy of diseases.

Macrophages are important in the regulation of immune responses. They pick up and ingest foreign materials and present these antigens to other cells of the immune system such as T cells and B cells. This is one of the important first steps in the initiation of an immune response.