6.1 Introduction to Immunity

The Immune System

The immune system is a complex and sophisticated defense mechanism that protects the human body from harmful pathogens, such as bacteria, viruses, and other foreign substances (Justiz-Vaillant et al., 2022). It is composed of various cells, tissues, and organs. These components work in harmony to identify, neutralize, and eliminate foreign invaders by distinguishing between self and non-self and recognizing specific patterns found on the surface of a pathogen.

There are two main branches of the immune system: the innate immune system and the adaptive immune system (as seen in Figure 6.2). The innate immune system provides the first line of defense and is always “on.” It acts rapidly upon encountering pathogens (Justiz-Vaillant et al., 2022). It includes physical barriers like the skin, hair, and mucous membranes as well as certain white blood cells, such as basophils, mast cells, monocytes, neutrophils, and macrophages, that respond quickly to infections.

When a pathogen breaches the innate defenses, the adaptive immune system (also known as the acquired immune system) comes into play to control infection. It is more specific to pathogens, has memory, and develops over time, through either encountering a pathogen or receiving a vaccination (Justiz-Vaillant et al., 2022).

Figure 6.2 This image illustrates the cooperation between the innate and the adaptive immune systems in response to pathogens. The innate immune system enhances adaptive immune responses so they can be more effective. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The adaptive immune system relies on white blood cells called T and B lymphocytes. T cells produce a cell-mediated immune response, and B cells produce the humoral immune response (as seen in Figure 6.3). These immune responses will be discussed in more detail in the subsequent sections.

Figure 6.3 This diagram illustrates T cell and B cell responses. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Antibody Mediated/Humoral Immunity

Antibody mediated/humoral immunity is a type of immune response that primarily involves B cells and their production of antibodies (Karagiannis & Arnold, 2022). When an antigen, such as a pathogen, foreign substance, or vaccination, enters the body, B cells recognize it and become activated (see Figure 6.3). These activated B cells then undergo differentiation and proliferation to form plasma cells, which are specialized factories for producing antibodies. The production of antibodies increases over time, reaching peak levels within 1 to 2 weeks after the initial encounter with the antigen. During the immune response, some B cells undergo differentiation into memory B cells. These cells remain in the body even after the antigen has been cleared. Memory B cells remember the encountered antigen and can rapidly initiate a strong response if the same antigen is encountered again in the future.

Cell-Mediated Immunity

Cell-mediated immunity is an immune response that involves T cells, specifically helper T cells (CD4 cells) and cytotoxic T cells (CD8 cells). This clusters of differentiation (CD) system is a way to classify and characterize different immune cell types based on the presence of specific surface markers, and this system is particularly relevant when distinguishing subsets of T helper cells, like CD4+ and CD8+ T cells. When an antigen is presented to T cells, helper T cells become activated and play a central role in coordinating the immune response (see Figure 6.3). They release chemical signals (cytokines) that stimulate the proliferation and activation of other immune cells, including cytotoxic T cells and B cells.

Cytotoxic T cells recognize cells that display foreign antigens on their surface, such as virus-infected cells or cancer cells. Cytotoxic T cells release toxic substances that attack and kill infected or abnormal cells, effectively eliminating the cells.

Antigen-Antibody Interactions

Antigen-antibody interactions are essential for the functioning of both innate and adaptive immune responses. They help in the identification and elimination of harmful invaders as well as in the establishment of immune memory, allowing the body to mount a more rapid and efficient response upon subsequent exposure to the same antigen. This process forms the basis for vaccination, where the body is exposed to harmless versions of antigens to develop immunity without experiencing severe symptoms of the disease.

Antigen-antibody interactions are fundamental processes in the immune response that play a crucial role in defending the body against infections and foreign substances (Karagiannis & Arnold, 2022). An antigen is a molecule or molecular structure, typically a protein or polysaccharide, that is recognized by the immune system as foreign or non-self. It can be present on the surface of pathogens such as bacteria, viruses, or parasites as well as on non-pathogenic substances like pollen or certain foods.

Antibodies, also known as immunoglobulins (Ig), are Y-shaped proteins produced by B cells in response to the presence of antigens (Aziz et al., 2023). Each antibody is specifically designed to recognize and bind to a particular antigen, much like a lock-and-key mechanism. The region of the antibody that binds to the antigen is called the antigen-binding site or paratope (Justiz-Vaillant et al., 2023; Greenspan, 2023).

When an antigen enters the body, it triggers the immune system to mount a response. B cells detect the antigen and start producing antibodies that specifically match its molecular structure. The process of antibody production and maturation takes a few days to weeks, but once antibodies are generated, they remain in the body as part of the immune memory.

The binding of antibodies to antigens is highly specific. When an antibody encounters its target antigen, it attaches to it, forming an antigen-antibody complex. This binding serves various purposes:

• Neutralization: Antibodies can neutralize pathogens or toxins by blocking their ability to infect cells or exert harmful effects.
• Opsonization: Antibodies can coat pathogens, facilitating their recognition and uptake by phagocytic cells like macrophages and neutrophils, leading to their destruction (Thau et al., 2023).
• Agglutination: Antibodies can bind to multiple antigens on the surface of pathogens, clumping them together and making it easier for phagocytes to engulf and clear them.
• Activation of complement: Antibodies can activate the complement system, a group of proteins that can lead to the destruction of pathogens through various mechanisms. Complement proteins interact like a chain reaction where one step leads to the next, creating a powerful response to invaders. This reaction is also known as a complement cascade (Bardhan & Kushik, 2023).