Immune system

A scanning electron microscope image of a single neutrophil (yellow/right), engulfing anthrax bacteria (orange/left) – scale bar is 5 µm (false color)
A scanning electron microscope image of normal circulating human blood. One can see red blood cells, several knobby white blood cells including lymphocytes, a monocyte, a neutrophil, and many small disc-shaped platelets.
Overview of the processes involved in the primary immune response
An antibody is made up of two heavy chains and two light chains. The unique variable region allows an antibody to recognize its matching antigen.
The time-course of an immune response begins with the initial pathogen encounter, (or initial vaccination) and leads to the formation and maintenance of active immunological memory.
Joints of a hand swollen and deformed by rheumatoid arthritis, an autoimmune disorder
Skeletal structural formula of the immunosuppressive drug dexamethasone
Polio vaccination in Egypt
Macrophages have identified a cancer cell (the large, spiky mass). Upon fusing with the cancer cell, the macrophages (smaller white cells) inject toxins that kill the tumor cell. Immunotherapy for the treatment of cancer is an active area of medical research.
Paul Ehrlich (1854–1915) was awarded a Nobel Prize in 1908 for his contributions to immunology.

Network of biological processes that protects an organism from diseases.

- Immune system

500 related topics


Complement system

Scheme of the complement system
Membrane Attack Complex (Terminal Complement Complex C5b-9)
Reaction Cascade of the Complement System: Classical, Alternative and Lectin Pathway, Amplification Loop, Terminal Pathway, and Membrane Attack Complex.
The classical and alternative complement pathways

The complement system, also known as complement cascade, is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane.


Process by which a cell uses its plasma membrane to engulf a large particle , giving rise to an internal compartment called the phagosome.

Overview of phagocytosis
Phagocytosis versus exocytosis
The engulfing of a pathogen by a phagocyte
Scanning electron micrograph of a phagocyte (yellow, right) phagocytosing anthrax bacilli (orange, left)
Trophozoites of Entamoeba histolytica with ingested erythrocytes

In a multicellular organism's immune system, phagocytosis is a major mechanism used to remove pathogens and cell debris.


Girl about to be vaccinated in her upper arm
In Sweden, polio vaccination started in 1957.
A mobile medicine laboratory providing vaccinations against diseases spread by ticks.
COVID-19 Vaccination Center of the Medical University of Gdańsk, Poland
Measles infection rate vs vaccination rate, 1980 - 2011. Source: WHO
Global smallpox cases from 1920 to 2010. Source: WHO (2011)
Share of children who receive key vaccines in 2016.
Global vaccination coverage 1980 to 2019 among one year olds
An 1802 testimonial to the efficacy of vaccination, presented to its pioneer, Edward Jenner, and signed by 112 members of the Physical Society, London.
A 1979 poster from Lagos, Nigeria, to promote the worldwide eradication of smallpox.
Vaccination rate by US state, including exemptions allowed by state in 2017
Global survey across 67 countries responding to the question: "Overall I think vaccines are safe". This image depicts the distribution of responses that replied "Strongly disagree" or "Tend to disagree" with the previous statement.
Dr Jenner performing his first vaccination on James Phipps, a boy of age 8. 14 May 1796. Painting by Ernest Board (early 20th century)
James Gillray's The Cow-Pock—or—the Wonderful Effects of the New Inoculation!, an 1802 caricature of vaccinated patients who feared it would make them sprout cowlike appendages
La vaccine or Le préjugé vaincu by Louis-Léopold Boilly, 1807
A doctor vaccinating a small girl, other girls with loosened blouses wait their turn apprehensively by Lance Calkin
German caricature showing von Behring extracting the serum with a tap.
Les Malheurs de la Vaccine (The history of vaccination seen from an economic point of view: A pharmacy up for sale; an outmoded inoculist selling his premises; Jenner, to the left, pursues a skeleton with a lancet)

Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease.


A scanning electron microscope image of a single neutrophil (yellow/right), engulfing anthrax bacteria (orange/left) – scale bar is 5 µm (false color)

Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent.

Humoral immunity

Aspect of immunity that is mediated by macromolecules - including secreted antibodies, complement proteins, and certain antimicrobial peptides - located in extracellular fluids.

Step 1: A macrophage engulfs the pathogen. Step 2: The macrophage then digests the bacterium and presents the pathogen's antigens. Step 3: A T helper cell binds to the macrophage and becomes an activated T helper cell. Step 4: The activated T helper cell binds to a B cell in order to activate the B cell. Step 5: When the B cells are activated, some B cells turn into plasma cells and are released in the blood, while other B cells become B memory cells that quicken response for a second exposure. Step 6: Plasma cells then secrete antibodies, which bind to antigens to fight the invading pathogens.
B cell activation is a large part of the humoral immune response.

The study of the molecular and cellular components that form the immune system, including their function and interaction, is the central science of immunology.

Cancer immunology

Tumor-associated immune cells in the tumor microenvironment (TME) of breast cancer models
Multiple factors determine whether tumor cells will be eliminated by the immune system or will escape detection. During the elimination phase immune effector cells such as CTL’s and NK cells with the help of dendritic and CD4+ T-cells are able to recognize and eliminate tumor cells.
Immune checkpoints of immunosuppressive actions associated with breast cancer

Cancer immunology is an interdisciplinary branch of biology that is concerned with understanding the role of the immune system in the progression and development of cancer; the most well known application is cancer immunotherapy, which utilises the immune system as a treatment for cancer.

Immunosuppressive drug


Immunosuppressive drugs, also known as immunosuppressive agents, immunosuppressants and antirejection medications, are drugs that inhibit or prevent activity of the immune system.


The human immunodeficiency viruses (HIV) are two species of Lentivirus (a subgroup of retrovirus) that infect humans.

Diagram of the HIV virion
A diagram of the HIV spike protein (green), with the fusion peptide epitope highlighted in red, and a broadly neutralizing antibody (yellow) binding to the fusion peptide
Structure of the RNA genome of HIV-1
Diagram of the immature and mature forms of HIV
The HIV replication cycle
Mechanism of viral entry: 1. Initial interaction between gp120 and CD4. 2. Conformational change in gp120 allows for secondary interaction with CCR5. 3. The distal tips of gp41 are inserted into the cellular membrane. 4. gp41 undergoes significant conformational change; folding in half and forming coiled-coils. This process pulls the viral and cellular membranes together, fusing them.
Clathrin-mediated endocytosis
Reverse transcription of the HIV genome into double-stranded DNA
HIV assembling on the surface of an infected macrophage. The HIV virions have been marked with a green fluorescent tag and then viewed under a fluorescent microscope.
Animation demonstrating cell-free spread of HIV
The phylogenetic tree of the SIV and HIV
A generalized graph of the relationship between HIV copies (viral load) and CD4 counts over the average course of untreated HIV infection; any particular individual's disease course may vary considerably.
Left to right: the African green monkey source of SIV, the sooty mangabey source of HIV-2, and the chimpanzee source of HIV-1

Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive.


Spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus.

The red ribbon is a symbol for solidarity with HIV-positive people and those living with AIDS.
Main symptoms of acute HIV infection
Main symptoms of AIDS.
CDC poster from 1989 highlighting the threat of AIDS associated with drug use
Diagram of a HIV virion structure
Scanning electron micrograph of HIV-1, colored green, budding from a cultured lymphocyte.
HIV replication cycle
A generalized graph of the relationship between HIV copies (viral load) and CD4<SUP>+</SUP> T cell counts over the average course of untreated HIV infection.
HIV Rapid Test being administered
AIDS Clinic, McLeod Ganj, Himachal Pradesh, India, 2010
People wearing AIDS awareness signs. on the left: "Facing AIDS a condom and a pill at a time"; on the right: "I am Facing AIDS because people I ♥ are infected"
Stribild – a common once-daily ART regime consisting of elvitegravir, emtricitabine, tenofovir and the booster cobicistat
Deaths due to HIV/AIDS per million persons in 2012
Trends in new cases and deaths per year from HIV/AIDS
The Morbidity and Mortality Weekly Report reported in 1981 on what was later to be called "AIDS".
Left to right: the African green monkey source of SIV, the sooty mangabey source of HIV-2, and the chimpanzee source of HIV-1
Ryan White became a poster child for HIV after being expelled from school because he was infected.
Changes in life expectancy in some African countries, 1960–2012

If the infection progresses, it interferes more with the immune system, increasing the risk of developing common infections such as tuberculosis, as well as other opportunistic infections, and tumors which are otherwise rare in people who have normal immune function.

Type 1 diabetes

A blue circle, the symbol for diabetes.
Overview of the most significant symptoms of diabetes

Type 1 diabetes (T1D), formerly known as juvenile diabetes, is an autoimmune disease that originates when cells that make insulin (beta cells) are destroyed by the immune system.