Immune System for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about the immune system, plus practice questions and answers.
(Note: This guide is part of our MCAT Biology series.)
Table of Contents
Part 1: Introduction
Part 2: Innate vs adaptive immunity
Part 3: Innate immune system
a) Anatomical barriers
b) Cellular component
c) Complement system
d) Cytokines
e) Inflammation
Part 4: Adaptive immune system
a) B cells and humoral immunity
b) T cells and cytotoxic immunity
Part 5: Anatomy of the immune system
a) Bone marrow
b) Spleen
c) Thymus
d) Lymphatic system
Part 6: Antigens and antibodies
a) Antibody structure
b) Antigen presentation
c) Recognition of self and non-self
Part 7: High-yield terms
Part 8: Passage-based questions and answers
Part 9: Standalone questions and answers
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Part 1: Introduction
The immune system is responsible for protecting the body against foreign invaders and diseases. It is a complex system that relies on constant feedback to develop immunity. There are 2 major components of the immune system that we will discuss: innate and adaptive immunity. We will also learn about other components of the body that aid in the immune response, as well as how immune system components eliminate diseases. Overall, this is a low to medium yield topic, but various aspects are regularly tested in passages and questions.
To gain an effective understanding of the immune system for the MCAT, you should first gain a basic understanding of the components of the immune system and how they operate, and then learn key principles likely to show up on the exam. As you work through this guide, it may be helpful for you to construct a series of flowcharts visualizing how each of these components relate to each other.
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Part 2: Innate vs adaptive immunity
The highest level of classification within the immune system is between the innate immune response and the adaptive immune response. These will be discussed in greater detail in the next two parts. In general, the innate immune response is composed of bodily components that non-selectively target diseases and foreign pathogens, while the adaptive immune response is selective to certain diseases. You are born with your innate immune response ready to go—the innate immune system can react to foreign pathogens on a time scale of seconds to hours. In contrast, the adaptive immune response takes much longer: on the time scale of days.
These two classifications can be divided even further into their constituent parts, which will be discussed in parts 3 and 4.
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Part 3: Innate immune system
As mentioned above, the innate immune system is responsible for general disease prevention and does not have the capability to recognize specific foreign particles or diseases. The innate immune system can be further divided into two main components: the non-cellular component and the cellular component. The non-cellular component consists of physical barriers from the body, as well as signaling molecules, while the cellular component consists of various types of white blood cells. The components of the innate immune system can operate individually to eliminate pathogens or work together in a process known as inflammation.
a) Anatomical barriers
The most prominent physical barrier from pathogens in the body is the skin. The skin is a tightly packed layer of cells that prevents materials from the outside from entering the body, greatly lowering the chance of disease. The skin also contributes to the innate immune response through its ability to sweat and shed the top layer of skin, thus sloughing off any pathogens that may be on it. In addition to providing a physical barrier against diseases, it contributes to the adaptive immune response by containing specific cellular components that take care of any pathogens that manage to enter the skin.
Other anatomical barriers are inside the body where pathogens can enter through the skin: such as the mouth and nose. The gastrointestinal tract is crucial in the innate immune response because bacteria and other pathogens can easily enter through the mouth. The first layer of defense is in the saliva, where the protein lysozyme helps break down bacterial cell walls.
The extremely acidic environment of the stomach also contributes to immune response. Since the pH is very acidic (close to 2), very few microorganisms can survive in the stomach. In addition to the breakdown of various pathogens, various components of the gastrointestinal tract aid in preventing disease. The mechanism of peristalsis—which moves food down the digestive tract—also keeps pathogens moving and prevents them from entering the body.
The naturally occurring gut flora in the gastrointestinal tract are also incredibly important to preventing bacterial infections. This may seem counterintuitive, but healthy bacteria prevent pathogenic bacteria from colonizing the body by outcompeting them and preventing them from spreading uncontrollably.
The respiratory system also contributes to innate immunity by filtering out pathogens and microparticles present in the air. Similar to the function of saliva, the mucus in the respiratory tract contains lysozyme to help kill bacteria. The mucus acts as a trap by preventing particles from flowing freely into the lungs. Mucus also works with cilia in the respiratory tract to push microbes into the pharynx, where they can then be swallowed into the acidic stomach and destroyed.
b) Cellular component
The cellular component of the innate immune response consists of white blood cells (leukocytes). There are many different types of white blood cells, with some contributing to the innate immune response and some contributing to the adaptive response. This section introduces a few types of leukocytes and describes their classifications.
Neutrophils are the most abundant type of white blood cells and phagocytose bacteria. They are often thought of as the first responder to an infection, so an elevated count of neutrophils is indicative of an infection. Lymphocytes are the second most common type of white blood cell, but not all lymphocytes are part of the innate immune response. There are 3 main types: B cells, T cells, and natural killer cells, and B and T cells contribute to the adaptive immune response and will be discussed in the next part of this guide. Natural killer cells, on the other hand, respond to cells infected by viruses and tumors to kill them and prevent the spread of infection. They are a somewhat specialized type of innate immune response because they target specific cells that have been tagged in some way.
Monocytes are the next most abundant type of white blood cell and travel to tissues throughout the body to differentiate into dendritic cells or macrophages. Dendritic cells serve as the bridge between the adaptive and immune responses by presenting antigens to T cells, a topic that will be discussed later. Macrophages are large cells analogous to garbage processors in the body, phagocytosing many pathogens and particles that don’t belong. Eosinophils are a type of white blood cell that targets parasitic infections. Basophils are a much smaller component of white blood cells and are involved in allergic responses, specifically through releasing histamine and heparin during an inflammatory response. Lastly, mast cells are similar to basophils except that they are found mostly in mucous membranes and connective tissues. Eosinophils, basophils, and mast cells may collectively be referred to as granulocytes, due to the presence of granules containing inflammatory components that can be seen under a microscope.
An important term to know for the MCAT is phagocyte, which refers to a cell that engages in phagocytosis (the engulfment and destruction of one cell by another). While macrophages engage in phagocytosis, it is important to know that macrophages are a subset of phagocytes responsible for engulfing pathogens, digesting pathogens, and presenting their fragments on the surface to communicate with other immune cells. We refer to these fragments as antigens. Broadly speaking, an antigen is a particle that triggers an immune response. Antigens can include but are not limited to: molecular fragments on the outside of bacterial walls, waste left behind by parasites, or allergens (substances that trigger an allergic reaction).
For more information on allergens, refer to the end of this guide.
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