How the immune system works

The immune system is an integrated network of cells, tissues, and organs that protects the body against pathogens, e.g. disease-causing agents such as viruses, bacteria, protozoa, fungi, and toxins. The immune system works against infectious agents and, sometimes, the body’s own cells.

Every cell in a person's body carries a set of surface proteins unique to that individual (except for identical twins, who share the same code) and these markers identify the cell as ‘self’. This protein set is known as the major histocompatibility complex. The prefix ‘histo’ means tissue. Major histocompatibility complex (MHC) is sometimes used interchangeably with HLA, referring to human leukocyte antigen.

Non-specific barriers to foreign organisms include the skin, mucous membranes, cilia, stomach acid, saliva, and tears. Cells and mechanisms of the innate immune system include phagocytes (so-called ‘eater’ cells that take up and degrade foreign micro-organisms), natural killer cells, and complement. Innate reactions are immediate, but do not confer enduring protective immunity against a pathogen. These cells can limit the spread of pathogens, but many pathogens have developed mechanisms with which to evade them.

Specific or acquired immunity comes once lymphocytes have developed antibodies against a particular pathogen. Natural active immunity is a result of infection, with the antibody marking the substance for destruction. Artificial active immunity can come from vaccines made from infectious agents or from minute microbe portions. When some lymphocytes are activated, they become ‘memory’ cells, providing a specific, faster, and enduring response to a particular pathogen when it is met again.

An antigen is any substance capable of provoking an immune response and antigens rest on the surface of the pathogen. Each antigen bears distinctive markers called epitopes. Because the MHC on the epitope is foreign, immune system cells are alerted and are able to organise a defence response. 

The immune system’s purpose is recognising and resisting any organism that does not carry the pattern of 'self' markers. This has a protective effect that is generally beneficial with some exceptions, notably organ transplant. Malfunctions of the immune system include allergic responses (hypersensitivity to a substance) and the development of autoimmune diseases (e.g. lupus, rheumatoid arthritis), in which the immune system attacks its own cells.

When the immune system is less active than it needs to be, infection is able to invade the body. If this happens on a recurring basis, the resulting condition is called immunodeficiency. The various cell types of the immune system are distributed through the body and play different roles; many moving about the body as needed.