The Cells of the Immune System
Multicellular organisms are a mosaic of different cells, tissues, and systems, each with their own specific function and interactions with other cells within the organism and/or the larger environment. The cells of the immune system are the guardians of these organisms – able to distinguish friend from foe and self from nonself. To accomplish this task in an organism with perhaps trillions of cells, the cells of the immune system must be ever vigilant and ready to respond at a moment’s notice. Many immune cells patrol other systems (such as the respiratory, digestive, and circulatory systems) and tissues as part of a broad surveillance system that seeks out invading pathogens or damaged cells, while other immune cells respond en masse when an alarm is raised.
Written by Jim Lane; Mahtomedi High School; Minnesota, USA
Background Image by Julien Resseguier; University of Oslo; Norway
Who’s Who? (Know Thyself)
The immune system has the immense challenge of determining who’s who within the vast ocean of cells of an organism. One way immune cells can determine the identity of a cell is with special membrane molecules called antigens. The type and number of antigens on a particular cell can identify the cell type and the species or individual to which it belongs. For example, the small yellow cells in this image are MRSA (methicillin-resistant Staphylococcus aureus), a kind of bacterium, surrounded by much larger red immune cells called neutrophils. The antigens on the surface of the MRSA cells allow the immune cells to identify them as foreign invaders, a pathogen. As you will learn below, immune cells use antigens to identify invading pathogens, and cells infected by them, to help protect and defend the body.
Image by Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, U.S. National Institutes of Health (NIH)
The Front Line
The immune system has two major components – an innate response and an adaptive response. The innate immune system is the front line of defense; it is constantly on patrol for nonself antigens and is ready to sound the alarm and respond to damaged cells or pathogenic invaders. The macrophage pictured here is one of the enforcers of the innate immune system as it surveys the identity of passing cells. If the macrophage finds a pathogen-associated (nonself) antigen or damage-associated antigen, it will phagocytize the other cell (carry out a form of cellular eating) and digest it. In a cruel twist, the macrophage will then display the antigens of the consumed cell as a signal to other macrophages and immune cells to detect and target the pathogen-associated antigens, triggering a much more specific immune response.
Image by National Institute of Allergy and Infectious Diseases, NIH
Targeted Attack
The macrophage has a close relative called a dendritic cell (in green), which can also display pathogen-associated antigens on its surface. But dendritic cells do not simply send a local message – they call in reinforcements and trigger a much more targeted and nuanced system-wide attack against invading pathogens. Dendritic cells do this by activating the adaptive immune response. The dendritic cells relay the message about an invasion by interacting and activating helper T cells (in pink).
Image by National Institute of Allergy and Infectious Diseases, NIH
All Hands on Deck
When dendritic cells sound an alarm, they activate a cascade of other immune cells to mount a full-blown response from the innate and adaptive immune systems. The dendritic cells activate helper T cells, which, in turn, activate B cells (pictured here). The B cells are antibody factories that target and disable pathogen-associated antigens. The antibodies target specific antigens and disable their function. The disabled pathogen is then quickly identified by macrophages or other immune cells and destroyed. Some B cells can turn into memory B cells with the ability to “remember” prior infections and respond rapidly to a reinfection by the same pathogen.
Image by National Institute of Allergy and Infectious Diseases, NIH
Know your Enemy
Helper T cells also activate the “special forces” of the immune system – killer T cells (pictured here). These killer cells are on a seek-and-destroy mission to destroy infected cells, preventing further spread of the pathogen and removing damaged cells. Natural kill cells help clear pathogens to restore normal homeostasis to the organism.
Image by National Institute of Allergy and Infectious Diseases, NIH
Remember the Face
The adaptive immune system remembers pathogen-associated antigens, creating lasting immunity from thousands of different pathogenic invaders. Vaccines use this ability to remember invaders to create immunity to a specific pathogen by introducing pathogen-associated antigens without triggering a full-blown infection. The helper T cells (pictured here) that activated the B cells and killer T cells can morph into memory T cells that are able to quickly reactivate and initiate an adaptive immune response in the event of a reinfection. And killer T cells can morph into memory killer T cells, which can quickly reactivate to attack and kill infected cells that display specific antigens. In this way, the adaptive immune system improves and refines its repertoire of known pathogens with each infection, creating an evolutionary arms race between pathogens and the immune system.
Image by National Institute of Allergy and Infectious Diseases, NIH
Stronger Together
The interplay between the innate and adaptive immune responses is the result of coordination and collaboration among dozens of different kinds of cells, each with their own action and reaction. Some cells activate others, some attack and destroy, others survey and remember prior attackers. Each relationship and interaction between cells is an integral part of the larger immune response. In this image, a larger blue dendritic cell is interacting with smaller red T regulatory cells. Regulatory T cells are responsible for reducing the immune response once the pathogen and the damaged cells have been removed, allowing for the organism to return to homeostasis, its normal state.
Image by National Institute of Allergy and Infectious Diseases, NIH
For suggestions on how to incorporate this journey into your teaching, see our “Implementation Suggestions.”