Macrophages are white blood cells that perform several crucial activities in the immune system. Although the usual function of macrophages is thought to be to promote non-specific, innate immunity, they also help to begin specific defense processes. These cells are vital to the inflammatory response, and can be induced to pursue single targets, such as tumor cells.
In the absence of foreign organisms like bacteria and viruses, one function of macrophages is to devour debris and dead tissue. Macrophages accomplish this task in the same way that they destroy foreign invaders, with a process called phagocytosis. During this process, the macrophage extends pseudopods to grab the object or organism, surrounds it, and takes it into its body inside of a vesicle. A structure called a lysosome then fuses to the vesicle, and destroys the object with enzymes and toxic chemicals.
After phagocytosis has been performed, another function becomes apparent. Molecules on the invader's surface that can be recognized by immune cells, known as antigens, are taken by the macrophage, and bound to a nearby helper T cell in a process known as "presentation." By binding the antigen to a specialized molecule on its own surface, the macrophage ensures that other white blood cells won't mistake it for an invader. If the helper T cell finds a matching antigen to the one it was presented by the macrophage, it will initiate an immune response.
Macrophages are also involved in specific immune responses when recruited by T cells. This macrophage function requires that the T cells release compounds known as lymphokines in response to tumor cells or infected somatic cells. These compounds bind the lymphokine receptors on the macrophage's surface, and activate the macrophage to attack nearby cells.
Another function of macrophages involves the inflammatory response. After tissue has been injured, macrophages in the area will release chemicals that promote blood flow to the region and cause inflammation. The inflammation, although painful, is necessary to ensure that other macrophages and immune cells can arrive to attack potential invaders and clear away dead cells.
Following an injury, a second wave of macrophages arrive about 48 hours later, that are not involved in phagocytosis or inflammation. These macrophages instead release a factor to promote tissue growth, repair, and differentiation in order to help recover from injury-related damage. The exact composition of this factor is not yet known, but tissue injured when deprived of macrophages tends to heal more slowly, providing evidence for its existence.