Understanding Immunity: Types, Mechanisms, and Vaccination
Understanding Immunity
What is Immunity?
Immunity is a medical term that describes the state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and nonspecific components.
Nonspecific Immunity
Nonspecific components act as barriers or eliminators of pathogens to ward off infection by microorganisms before they can cause disease.
Specific (Adaptive) Immunity
Other components of the immune system adapt to each new disease encountered and generate specific immunity against the pathogen. Adaptive immunity is often subdivided into two main types: active and passive. These can be further subdivided based on duration. Passive immunity is short-term (a few months), while active immunity is long-term and sometimes lifelong.
Types of Adaptive Immunity
- Humoral Immunity: Mediated by secreted antibodies.
- Cellular Immunity: Protection provided by T cells.
Humoral immunity is active when the body produces its own antibodies and passive when antibodies are transferred between individuals. Similarly, cellular immunity is active when the body’s own T cells are stimulated and passive when T cells come from another organism.
Passive Immunity
Passive immunity is the transfer of active immunity, in the form of antibodies, from one individual to another. This can occur naturally (e.g., maternal antibodies transferred to the fetus through the placenta) or artificially (e.g., high levels of antibodies specific for a pathogen or toxin transferred to non-immune individuals).
Naturally Acquired Passive Immunity
Maternal passive immunity is a naturally acquired type, where maternal antibodies are passed to the fetus through the placenta around the third month of gestation. IgG is the only antibody isotype that can cross the placenta. Passive immunity is also provided through the transfer of immunoglobulin antibodies found in breast milk, protecting the newborn against bacterial infections.
Artificially Acquired Passive Immunity
Artificially acquired passive immunity is short-term immunization induced by the transfer of antibodies. This can be administered through human or animal blood plasma, intravenous or intramuscular human immunoglobulin, and monoclonal antibodies. Passive transfer is used prophylactically for immunodeficiency diseases like hypogammaglobulinemia. It’s also used to treat various acute infections and poisoning. Protection is short-lived, and there are risks of hypersensitivity reactions and serum sickness, especially with non-human gamma globulin.
Artificial passive immunity has been used for over a century to treat infectious diseases. Before antibiotics, it was often the only specific treatment for certain infections. Immunoglobulin therapy remained a first-line treatment for severe respiratory diseases until the 1930s, even after antibiotics and sulfonamides were introduced.
Active Immunity
When B cells and T cells are activated by a pathogen, memory B cells and memory T cells develop. These memory cells remember each specific pathogen encountered and mount a strong response if the pathogen is detected again. This type of immunity is both active and adaptive because the immune system prepares itself for future challenges. Active immunity often involves both cellular and humoral immunity, as well as the innate immune system.
Naturally Acquired Active Immunity
This occurs when a person is exposed to a live pathogen and develops a primary immune response, leading to immunological memory. This type of immunity is natural because it isn’t medically induced. Immunodeficiency (both acquired and congenital) and immunosuppression can affect the formation of active immunity.
Artificially Acquired Active Immunity
This type of immunity is induced by a vaccine—a substance containing an antigen. A vaccine stimulates a primary immune response against the antigen without causing disease symptoms. Edward Jenner coined the term “vaccination,” later adapted by Louis Pasteur for his pioneering work. Pasteur treated infectious agents to weaken them, so they couldn’t cause severe illness.
Types of Traditional Vaccines
- Inactivated vaccines: Killed microorganisms (e.g., flu, cholera, plague, hepatitis A). Often require booster shots.
- Live attenuated vaccines: Weakened microorganisms (e.g., yellow fever, measles, rubella, mumps). Provide more durable responses and generally don’t require boosters.
- Toxoids: Inactivated toxic compounds from microorganisms (e.g., tetanus, diphtheria).
- Subunit vaccines: Small fragments of disease-causing organisms (e.g., hepatitis B).
Most vaccines are administered via hypodermic injection because they aren’t reliably absorbed through the digestive system. Live attenuated vaccines for polio and some for typhus and cholera are administered orally to produce immunity in the intestine.