These cells are the body's main defense against bacteria. When a neutrophil ingests a microbe, the cell's toxic chemicals kill and digest it. Monocytes are another type of white blood cell. They circulate in the blood and swallow any invaders that they encounter. Monocytes may leave the bloodstream and migrate into the tissues, where they become macrophages .
Specialized macrophages protect the intestines, lungs, liver and brain.
Monocytes and macrophages ingest and break up microbes or cancerous cells and display pieces of them on their surface, where they can be recognized by helper T-cells. For this reason, they are known as antigen-presenting cells . Antigens and Antibodies Antigens are proteins that stimulate an immune response. Antibodies are made up of pieces that can combine in many different ways. This enables the immune system to recognize and defend against many different foreign invaders.