Humoral immunity is a part of the immune system that involves producing and circulating antibodies. Antibodies are the proteins produced by specialized white blood cells called B cells (B lymphocytes). These antibodies are also known as immunoglobulins.
This immune response is primarily responsible for defending the body against extracellular pathogens, such as bacteria and viruses, that circulate in bodily fluids like blood and lymph. When B cells encounter foreign substances, known as antigens, they undergo a process of activation and differentiation. This process produces plasma cells, which are responsible for secreting antibodies into the bloodstream.
Antibodies play an essential role in humoral immunity because they recognize and bind to specific antigens on the surface of pathogens. The binding can neutralize the pathogens directly or mark them for destruction by other immune system components, such as phagocytes. Additionally, antibodies can activate the complement system.
Humoral immunity is one of the two main branches of the adaptive immune system. The other being cell-mediated immunity, which involves the activation of T cells and focuses more on combating intracellular pathogens. Together, these components provide a comprehensive defense against various infectious agents.
Table of Contents
Stages of Humoral Immunity
Humoral immunity involves several stages in response to an infection or the introduction of foreign substances (antigens). These stages collectively contribute to the humoral immune response, crucial for defending the body against extracellular pathogens. These stages include:
- Antigen Recognition: The process begins when B cells encounter antigens, typically proteins or other molecules on the surface of pathogens. B cells have specific receptors on their surface that can bind to these antigens.
- Activation of B Cells: Once B cells recognize an antigen that matches their receptors, they become activated. This activation often requires assistance from helper T cells, which release signaling molecules (cytokines) to stimulate the B cell.
- Proliferation and Differentiation: Activated B cells undergo rapid proliferation, leading to the formation of a clone of identical B cells. Most of these cells differentiate into plasma cells responsible for antibody production.
- Antibody Production: Plasma cells secrete large quantities of antibodies into the bloodstream. Antibodies are Y-shaped proteins that specifically bind to the antigens on the surface of pathogens, marking them for destruction or neutralization.
- Antibody Circulation: Antibodies circulate in the bloodstream and other bodily fluids, reaching various tissues to encounter and neutralize pathogens. They can also bind to toxins and prevent them from harming the body.
- Memory B Cell Formation: Some activated B cells differentiate into memory B cells. These cells “remember” the specific antigen they encountered, providing a quicker and more robust response upon subsequent exposure to the same antigen. This is the basis for immunological memory.
- Antibody-Mediated Effector Functions: Antibodies carry out several effector functions, including neutralization (blocking the harmful effects of pathogens), opsonization (marking pathogens for destruction by phagocytes), and activation of the complement system (a group of proteins that enhance the immune response).
- Resolution and Long-Term Immunity: The immune response gradually decreases as the infection clears. However, memory B cells persist, providing long-term immunity. If the individual encounters the same antigen in the future, the immune system can mount a rapid and effective response.
Types of Humoral immunity with Examples
Humoral immunity can be categorized into two primary types: Active and passive. The basis of this differentiation is how one acquires the immune response. These two types of humoral immunity are essential in defending the body against infections.
Active immunity offers long-term protection and immunological memory, while passive immunity provides immediate but temporary protection. The choice between active and passive immunity depends on factors such as the nature of the threat, the urgency of protection, and the ability of the individual’s immune system to respond effectively.
Active Humoral Immunity
Active humoral immunity refers to the immune response where the activation of the body’s immune system produces antibodies in response to exposure to antigens. This exposure can be either natural, through infection with a pathogen, or artificial, through vaccination.
- Natural Active Humoral Immunity:
- Example 1: Bacterial Infection: If an individual is infected with a bacterium, such as Streptococcus pneumoniae (causing pneumonia), the immune system recognizes the bacterial antigens. B cells activate to produce antibodies specific to these antigens. This immune response eliminates the bacteria and establishes immunological memory so that the body can mount a quicker and more effective response upon re-exposure.
- Example 2: Viral Infection: In the case of a viral infection, like influenza, the immune system recognizes viral antigens. B cells activate to produce antibodies that can neutralize the virus and prevent its entry into host cells. These antibodies can also mark the virus for destruction by other immune cells.
- Artificial Active Humoral Immunity:
- Example 1: Vaccination against Measles: A person receives a measles vaccine containing weakened or inactivated measles virus antigens. The immune system perceives the antigens as foreign and mounts an immune response. Then, B cells activate to produce antibodies specific to the measles virus. If the vaccinated person is later exposed to the measles virus, their immune system can rapidly produce antibodies, protecting against the infection.
- Example 2: Hepatitis B Vaccination: Hepatitis B vaccine contains a protein from the surface of the hepatitis B virus. When an individual receives the vaccine, their immune system recognizes the viral protein as foreign and produces antibodies against it. This immune response protects against future infections with the hepatitis B virus.
In both types of active humoral immunity, the key is activating the immune system to produce antibodies and establish immunological memory. The memory allows the immune system to respond more rapidly and effectively upon subsequent exposure to the same pathogen, providing long-lasting protection against specific infections.
Passive Humoral Immunity
Passive humoral immunity involves the transfer of pre-formed antibodies from one individual to another. This transfer can occur naturally or artificially, providing immediate but temporary protection against specific pathogens.
- Natural Passive Humoral Immunity:
- Example 1: Maternal Antibodies: During pregnancy, a pregnant woman can transfer antibodies, primarily IgG, across the placenta to her developing fetus. These maternal antibodies provide the newborn with temporary protection against certain infections during the early months of life. This passive transfer helps protect the infant until its immune system matures and produces antibodies.
- Example 2: Breast Milk Antibodies: Breast milk contains antibodies, predominantly IgA and other immune components. When a baby is breastfed, it receives these antibodies, providing additional protection against infections in the gastrointestinal and respiratory tracts. This natural passive immunity is crucial for the newborn’s defense against common pathogens.
- Artificial Passive Humoral Immunity:
- Example 1: Immune Globulin Administration: Immune globulin, which contains a mixture of antibodies, can be administered to individuals for immediate protection against certain diseases. This can be useful when there is a requirement for rapid immunity, such as exposure to a known pathogen. For example, individuals may receive immune globulin-containing antibodies against hepatitis A or rabies after potential exposure.
- Example 2: Antivenom Treatment: In cases of snake or spider bites, individuals may receive antivenom, a preparation containing antibodies against the venom. This provides immediate passive immunity, neutralizing the venom and preventing or minimizing the effects of envenomation. The antibodies in antivenom are typically derived from animals that have been immunized with venom.
Passive humoral immunity offers quick but short-lived protection because the transferred antibodies eventually degrade over time, and immunological memory is absent.. In contrast to active immunity, where the individual’s immune system generates antibodies, passive immunity directly transfers antibodies from an external source.
Difference Between Active and Passive Humoral Immunity
Here’s a table summarizing the differences between active and passive humoral immune responses:
|Active Immune Response
|Passive Immune Response
|Nature of Immunity
|Source of Antibodies
|Generated by the individual’s immune system
|Transferred from an external source
|Takes time to develop (days to weeks)
|Generates immunological memory
|Does not generate immunological memory
|Vaccination or infection
|Maternal antibodies transferred to fetus/infant
These are the primary distinctions between active and passive humoral immune responses.
- Janeway CA Jr, Travers P, Walport M, et al. Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science; 2001. Chapter 9, The Humoral Immune Response. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10752/
- Image source: Urry, Lisa A., et al. Campbell Biology. Pearson Higher Education, Inc., 2018.
- Owen, J. A., Punt, J., & Stranford, S. A. (2013). Immunology (7th ed.). Macmillan Higher Education.