Review of Adaptive Immunity: Videos & Practice Problems

Adaptive immunity involves the maturation of T cells in the thymus and B cells in the bone marrow, followed by their activation in secondary lymphoid organs. Naive T cells require antigen presentation by antigen-presenting cells like dendritic cells, utilizing MHC class I and II. Activated T cells proliferate and differentiate into effector or memory cells, with cytotoxic T cells inducing apoptosis in infected cells, while helper T cells activate B cells and macrophages. Activated B cells produce antibodies, crucial for targeting pathogens, highlighting the specificity and memory aspects of the immune response.

Review of Adaptive Immunity

Adaptive immunity is a crucial aspect of the immune system, characterized by its ability to recognize and respond to specific pathogens. This process begins in the primary lymphoid organs, namely the thymus and bone marrow, where T cells and B cells mature. Once fully developed, these cells migrate to secondary lymphoid organs, where they exist in their naive forms, meaning they are inactive and require activation to mount an immune response.

Activation of T cells occurs when they encounter their specific antigen presented on Major Histocompatibility Complex (MHC) molecules by antigen-presenting cells, such as dendritic cells. Dendritic cells can present antigens using MHC class I for cytotoxic T cells (TC cells) and MHC class II for helper T cells (TH cells). This interaction is essential for T cell activation, which is further enhanced by co-stimulatory signals from the dendritic cells.

Upon activation, T cells proliferate and differentiate into various cell types, including effector cells and memory cells. Effector cytotoxic T cells are responsible for inducing apoptosis in infected cells, thereby limiting pathogen spread. In contrast, helper T cells play a pivotal role in activating other immune components, including B cells and macrophages, through the release of cytokines. This activation enhances the immune response, enabling macrophages to eliminate pathogens more effectively.

B cells also require activation, which occurs when they bind to their specific antigen. After internalizing and processing the antigen, B cells present it on MHC class II molecules, allowing activated helper T cells to stimulate them. Activated B cells proliferate and can differentiate into plasma cells, which secrete antibodies, or memory B cells, which are vital for a quicker response to future infections. Plasma cells can produce thousands of antibodies per second, facilitating various immune responses, such as tagging pathogens for destruction.

A key feature of adaptive immunity is its memory component, which allows for a more robust response upon re-exposure to the same antigen. This specificity means that each T cell and B cell is tailored to respond to a unique antigen, highlighting the importance of a diverse repertoire of these cells to effectively combat a wide range of pathogens. Overall, the adaptive immune system's ability to remember past infections and respond specifically is fundamental to its effectiveness in protecting the body.

Why would a person who has their tonsils removed be more susceptible to certain types of infections of the throat and respiratory tract?

• A. Tonsils produce high levels of lactoferrin, a strong natural antibacterial compound.
• B. Tonsils produce large amounts of interferons, natural antiviral compounds.
• C. Tonsils are secondary lymphoid organs where immune cells aggregate providing easy immune response to oral microbes.
• D. Tonsils are the location where T cells develop and mature. Without tonsils, the T cells would not be able to fully mature and the individual’s immune system functions would suffer.

Answer: (C)

How are T cell receptors similar in function to B cell receptors?

• A. Both receptors are composed of two chains, a heavy and a light chain.
• B. Both receptors bind epitopes (small unique sections of antigen molecules).
• C. Both bind structures directly on the surface of microbes.
• D. Both can be secreted from lymphocytes to bind to extracellular pathogens.

Answer: (B)

Which of the following is not typical of an immunogenic antigen?

• A. Viral protein.
• B. Foreign nucleic acid.
• C. Molecules not made by the host organism.
• D. Cytoplasmic proteins commonly found in host skin cells.

Answer: (D)

In opsonization with IgG, why would it be important that IgG react with the antigen BEFORE a phagocytic cell recognizes the antibody molecule?

• A. If the IgG is bound to the phagocyte before opsonization, it would most likely be ingested by the phagocyte before it could bind to a pathogen.
• B. Binding of IgG by phagocytes would block the antigen binding sites on the IgG molecules, preventing them from binding to the microbes.
• C. Binding of IgG by phagocytes changes the conformation of the antibody’s antigen binding site making it less effective at binding the correct foreign antigen.
• D. Binding of an antibody by phagocytes results in immediate release of damaging hydrolytic enzymes to the outside of the cell which would destroy all remaining antibodies.

Answer: (A)

Here’s what students ask on this topic:

What are the primary lymphoid organs and their roles in adaptive immunity?

The primary lymphoid organs are the thymus and bone marrow. The thymus is where T cells fully develop and mature, while the bone marrow is where B cells fully develop and mature. These organs are crucial for the initial development of the immune cells that will later participate in adaptive immunity. Once matured, T cells and B cells migrate to secondary lymphoid organs where they can encounter antigens and become activated to generate an immune response.

How do naive T cells become activated in adaptive immunity?

Naive T cells become activated through antigen presentation by antigen-presenting cells (APCs) like dendritic cells. The dendritic cells present antigens on their surface using MHC class I and II molecules. MHC class I presents to cytotoxic T cells, while MHC class II presents to helper T cells. This interaction, along with co-stimulatory signals, activates the naive T cells, allowing them to proliferate and differentiate into effector and memory T cells.

What is the role of helper T cells in adaptive immunity?

Helper T cells play a crucial role in adaptive immunity by activating other immune cells. Once activated by antigen-presenting cells, helper T cells can proliferate and differentiate into effector and memory cells. Effector helper T cells release cytokines that activate B cells and macrophages. Activated B cells produce antibodies, while activated macrophages have increased killing power to eliminate pathogens. This coordination enhances the overall immune response.

What is the significance of memory cells in adaptive immunity?

Memory cells are a vital component of adaptive immunity because they provide a faster and more robust response upon re-exposure to the same antigen. Both memory T cells and memory B cells are generated during the initial immune response. These cells remain in the body and can quickly recognize and respond to the same antigen in future infections, leading to a more efficient and effective immune response.

How do B cells contribute to adaptive immunity?

B cells contribute to adaptive immunity by producing antibodies. When a naive B cell encounters its specific antigen, it binds to the antigen, internalizes it, processes it, and presents it on MHC class II molecules. Helper T cells recognize this presentation and activate the B cell. The activated B cell then proliferates and differentiates into plasma cells and memory B cells. Plasma cells secrete large quantities of antibodies that tag pathogens for removal, while memory B cells provide long-term immunity.