Understanding Immunity

A breakdown of immunity and how it works

Reviewed by: Teresa Coats, MD, FACP, and Shelley Payne, PhD, LaMontagne Center for Infectious Disease
Video by: Jeshua Mauldin
Written by: Ashley Lawrence

Many of us know that immunity is the body’s natural defense against infections, such as the common cold and flu. But what exactly is the immune system and how does immunity work? Let’s break it down…

What is the immune system?

Your immune system is your body’s defense system against foreign pathogens (bacteria, viruses, or other microorganisms that can cause disease), and it is made up of a complex network of organs, cells, and chemicals. The parts of the immune system that actively fight against infection and foreign invaders are white blood cells, antibodies, the complement system, the lymphatic system, the spleen, the thymus, and the bone marrow.

Phagocytes and lymphocytes are types of white blood cells that play a key role in fighting infection:

  • Phagocytes are white blood cells that engulf and absorb harmful bacteria, particles, and dead or dying cells.
  • Lymphocytes are white blood cells that generate immune responses.

There are two main types of lymphocytes:

  • B lymphocytes make antibodies.
  • T lymphocytes help control immune responses and help fight pathogens, especially viruses, that grow inside our cells.

Learn more about ways you can support your immune system.

<br>How does immunity work?

When your body senses foreign pathogens, the immune system works to recognize those foreign invaders and get rid of them. B lymphocytes are triggered to make antibodies, which lock onto foreign pathogens. The antibodies may block the pathogen from invading your cells or tag the pathogen for destruction by phagocytes. Some T lymphocytes (“killer” T cells) protect us against viruses and other pathogens that grow inside our cells by killing the infected cells. This prevents the intracellular pathogens from replicating and escaping to infect more cells. Both B and T lymphocytes develop a memory compartment (known as immunological memory), keeping a record of the foreign pathogens they have encountered in your lifetime so they can quickly recognize and destroy the foreign pathogen as it enters your body again. This prevents the foreign pathogen from multiplying and making you ill.

What are the different types of immunity?

Innate Immunity

Innate (or natural) immunity consists of the physical, chemical, and cellular defenses you are born with. Your innate immune system is activated immediately, preventing the spread and movement of foreign pathogens throughout your body.

Physical Barriers

  • Skin
  • Cilia, eyelashes, and other body hair

Defense Mechanisms

  • Secretions
  • Mucous
  • Bile
  • Gastric acid
  • Saliva, tears, and sweat

Adaptive Immunity

Adaptive (or active) immunity develops during your lifetime through immunological memory. Your adaptive immune system recognizes and remembers specific foreign pathogens, providing defense and protection against recurrent infections. When your adaptive immune system is exposed to a new threat, the specifics of the foreign pathogen are memorized, preventing you from getting the infection again for a duration of time.


Vaccines are an example of how immunological memory works. A vaccination can be made using either an active, but weakened, foreign pathogen or by using specific parts of a foreign pathogen that are not active—neither of which causes an active infection. Instead, the vaccination mimics the presence of an active foreign pathogen to trigger an immune response, even though no real threats are present. When you get a vaccination, your body produces antibodies and T cells that recognize that foreign pathogen. Then, your adaptive immune system memorizes the foreign substance and it is ready to protect you when you are exposed to the actual pathogen—all without you having to actually experience the illness. Because immunity is specific, your body may not be able to recognize closely related pathogens. There are many different viruses that cause the common cold, so if you have been infected with one, you will have immunity to it, but you can still get infected with another cold virus that your immune system hasn’t seen before. New flu viruses circulate almost every year, so we get a new vaccination every year to protect us against the new strains.

Passive Immunity

Passive (or borrowed) immunity is a short-term form of immunity, in which antibodies are transferred to you from another source. Your passive immune system can develop naturally or artificially.


  • Antibodies received from mother (through the placenta or breast milk)


  • Antibodies received from a donor (gamma globulin injection or infusion)

What do we know about COVID-19?

There are many uncertainties surrounding COVID-19. “A significant problem with determining whether infection with SARS-Co-2 (the virus strain that causes COVID-19) is protective against future infection is that we do not know, yet, how long antibodies last and whether these antibodies are able to neutralize (or block) the infection,” explains Teresa Coats, MD, FACP, an internal medicine specialist in UT Health Austin’s Primary Care Clinic. “It is also not clear how reliable the various antibody tests are. Projections based on information from other coronaviruses are hopeful, though, and research is ongoing to attain better information.”

The COVID-19 outbreak is a rapidly evolving situation, and information will be updated as it becomes available. You can stay informed by visiting the Centers for Disease Control and Prevention (CDC) website.

About UT Health Austin

UT Health Austin is the clinical practice of the Dell Medical School at The University of Texas at Austin. We collaborate with our colleagues at the Dell Medical School and The University of Texas at Austin to utilize the latest research, diagnostic, and treatment techniques, allowing us to provide patients with an unparalleled quality of care. Our experienced healthcare professionals deliver personalized, whole-person care of uncompromising quality and treat each patient as an individual with unique circumstances, priorities, and beliefs. Working directly with you, your care team creates an individualized care plan to help you reach the goals that matter most to you — in the care room and beyond. For more information, call us at 1-833-UT-CARES or request an appointment here.