An Introduction to Active Immunity and Passive Immunity

Overview of Immunity

One way to categories immunity is as nonspecific and specific.

• Nonspecific defenses: These defenses work against all foreign matter and pathogens. Examples include physical barriers, such as mucous, nasal hair, eyelashes, and cilia. Chemical barriers are also a type of nonspecific defense. Chemical barriers include the low pH of the skin and gastric juice, the enzyme lysozyme in tears, the alkaline environment of the vagina, and earwax.
• Specific defenses: This line of defenses is active against particular threats, such as particular bacteria, viruses, fungi, prions, and mold. A specific defense that acts against one pathogen usually isn't active against a different one. An example of specific immunity is resistance to chickenpox, either from exposure or a vaccine.

Another way to group immune responses is:

• Innate immunity: A type of natural immunity that is inherited or based on genetic predisposition. This type of immunity confers protection from birth until death. Innate immunity consists of external defenses (the first line of defense) and internal defenses (second line of defense). Internal defenses include fever, the complement system, natural killer (NK) cells, inflammation, phagocytes, and interferon. Innate immunity is also known as genetic immunity or familial immunity.
• Acquired immunity: Acquired or adaptive immunity is the body's third line of defense. This is protection against specific types of pathogens. Acquired immunity may be either natural or artificial in nature. Both natural and artificial immunity have passive and active components. Active immunity results from an infection or an immunization, while passive immunity comes from naturally or artificially gaining antibodies.

Let's take a closer look at active and passive immunity and the differences between them.

Active Immunity

Activite immunity comes from exposure to a pathogen. Surface markers on the pathogen surface act as antigens, which are binding sites for antibodies. Antibodies are Y-shaped protein molecules, which can exist on their own or attach to the membrane of special cells. The body doesn't keep a store of antibodies on hand to take down an infection immediately. A process called clonal selection and expansion builds up sufficient antibodies.

Examples of Active Immunity

An example of natural activity immunity is fighting off a cold. An example of artificial active immunity is building up a resistance to a disease due to immunization. An allergic reaction is an extreme response to an antigen, resulting from active immunity.

Features of Active Immunity

• Active immunity requires exposure to a pathogen or to the antigen of a pathogen.
• Exposure to the antigen leads to the production of antibodies. These antibodies essentially mark a cell for destruction by special blood cells called lymphocytes.
• Cells involved in active immunity are T cells (cytotoxic T cells, helper T cells, memory T cells, and suppressor T cells), B cells (memory B cells and plasma cells), and antigen-presenting cells (B cells, dendritic cells, and macrophages).
• There is a delay between exposure to the antigen and acquiring immunity. The first exposure leads to what is called a primary response. If a person is exposed to the pathogen again later, the response is much faster and stronger. This is called a secondary response.
• Active immunity lasts a long time. It can endure for years or an entire life.
• There are few side effects of active immunity. It can be implicated in autoimmune diseases and allergies, but generally doesn't cause problems.

Passive Immunity

Passive immunity doesn't require the body to make antibodies to antigens. The antibodies are introduced from outside the organism.

Examples of Passive Immunity

An example of natural passive immunity is a baby's protection against certain infections by getting antibodies through colostrum or breast milk. An example of artificial passive immunity is getting an injection of antisera, which is a suspension of antibody particles. Another example is the injection of snake antivenom following a bite.

Features of Passive Immunity

• Passive immunity is conferred from outside the body, so it doesn't require exposure to an infectious agent or its antigen.
• There is no delay in the action of passive immunity. Its response to an infectious agent is immediate.
• Passive immunity is not as long-lasting as active immunity. It is typically only effective for a few days.
• A condition called serum sickness can result from exposure to antisera.