Immune System: Parts of the Immune System
While the immune system has organs and tissues like other parts of the body, the easiest way to think about this system is by how it generates immune responses. In this regard there is an innate and adaptive immune response.
Innate immune response
This part of the immune system can be considered our first line of defense. It fends off the majority of pathogens, and it is quick. The way this part of the immune system defends us is by physical barriers (like our skin), chemical barriers (like mucus), and cells that are non-discerning, meaning that they destroy without worrying about what is harmed, including our own cells. Important innate immune system cells include neutrophils, macrophages, and natural killer (NK) cells. Another cell type in the innate immune system cell is the dendritic cell, which is the most important “messenger” cell to alert the adaptive immune system.
Watch this short animation about the innate immune system.
Adaptive immune response
This part of the system can be considered the “special forces.” If the innate immune system is not able to stop the infection, the components of this part of the immune response are activated and have two important jobs:
- Stop the infection
- Generate immunologic memory
The two most important cell types that respond are T cells and B cells.
T cells
T cells ensure that the immune response is strong enough, but not too strong. Three types of T cells have different jobs:
- Helper T cells — work to activate other cells of the immune response, particularly B cells
- Cytotoxic T cells — kill cells infected by viruses
- Regulatory T cells — regulate the immune response
B cells
B cells produce antibodies to neutralize the infection. Some B cells also become memory cells. Memory cells circulate in our bodies at low levels after an infection (or vaccination) and monitor for any future infection by the same pathogen. Memory T cells also remain. If memory cells are activated by the pathogen in the future, they allow for a faster and stronger immune response. Often, we never know we were exposed a second (or third or fourth, etc.) time. Vaccination leverages immunologic memory by preparing our bodies for subsequent infections.
- Watch this short animation about the adaptive immune system.
- Watch this short animation to see how antibodies work.
- Find out more about the parts of the immune system on the VEC website.
Reviewed by Paul A. Offit, MD, on Nov. 20, 2025