PCAT Exam: Biological Processes - Immunity

The immune system is the body’s defense against invading microorganisms (bacteria, viruses, fungi, and parasites) and other harmful, foreign substances. It is capable of limiting or preventing infection.

There are two general types of immunity: innate immunity and acquired immunity.

Innate immunity uses physical and chemical barriers to block the entry of microorganisms into the body. The skin forms a physical barrier that blocks microorganisms from entering underlying tissues. Mucous membranes in the digestive, respiratory, and urinary systems secrete mucus to block and remove invading microorganisms. Saliva, tears, and stomach acids are examples of chemical barriers intended to block infection with microorganisms. In addition, macrophages and other white blood cells can recognize and eliminate foreign objects through phagocytosis or direct lysis.

Acquired immunity refers to a specific set of events used by the body to fight a particular infection. Essentially, the body accumulates and stores information about the nature of an invading microorganism. As a result, the body can mount a specific attack that is much more effective than innate immunity. It also provides a way for the body to prevent future infections by the same microorganism.

Acquired immunity is divided into a primary response and a secondary response.

The primary immune response occurs the first time that a particular microorganism enters the body, where macrophages engulf the microorganism and travel to the lymph nodes. In the lymph nodes, macrophages present the invader to helper T lymphocytes, which then activate humoral and cellular immunity. Humoral immunity refers to immunity resulting from antibody production by B lymphocytes. After being activated by helper T lymphocytes, B lymphocytes multiply and divide into plasma cells and memory cells. Plasma cells are B lymphocytes that produce immune proteins called antibodies, or immunoglobulins. Antibodies then bind to the microorganism to flag it for destruction by other white blood cells. Cellular immunity refers to the immune response coordinated by T lymphocytes. After being activated by helper T lymphocytes, other T lymphocytes attack and kill cells that cause infection or disease.

The secondary immune response takes place during subsequent encounters with a known microorganism. Memory cells respond to the previously encountered microorganism by immediately producing antibodies. Memory cells are B lymphocytes that store information to produce antibodies. The secondary immune response is swift and powerful, because it eliminates the need for the time-consuming macrophage activation of the primary immune response. Suppressor T lymphocytes also take part to inhibit the immune response, as an overactive immune response could cause damage to healthy cells.

Active and Passive Immunity
Immunization is the process of inducing immunity. Active immunization refers to immunity gained by exposure to infectious microorganisms or viruses and can be natural or artificial. Natural immunization refers to an individual being exposed to an infectious organism as a part of daily life. For example, it was once common for parents to expose their children to childhood diseases such as measles or chicken pox. Artificial immunization refers to therapeutic exposure to an infectious organism as a way of protecting an individual from disease. Today, the medical community relies on artificial immunization as a way to induce immunity.

Vaccines are used for the development of active immunity. A vaccine contains a killed, weakened, or inactivated microorganism or virus that is administered through injection, by mouth, or by aerosol. Vaccinations are administered to prevent an infectious disease, but they do not always guarantee immunity.

Passive immunity refers to immunity gained by the introduction of antibodies. This introduction can be natural or artificial. The process occurs when antibodies from the mother’s bloodstream are passed on to the bloodstream of the developing fetus. Breast milk can also transmit antibodies to a baby. Babies are born with passive immunity, which provides protection against general infection for approximately the first six months of life.