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.
Vaccines work by stimulating the immune system to produce antibodies.
Antibodies coat foreign agents and mark them for destruction. They alert immune system cells such as killer T-cells and macrophages to attack the invader. Antibodies also activate the complement system , a complex cascade of chemical reactions that destroys bacteria by punching holes in their membranes. Natural killer cells are less particular and attack a variety of enemies. B-cells are produced and mature in the bone marrow.