Overview of the adaptive immune system

Question 1

Describe antigen presentation as one of the main links between the innate and the adaptive immune responses

Answer 1

Antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells process and present antigenic peptides on their surface in the context of major histocompatibility complex (MHC) molecules.

These MHC-peptide complexes can be recognised by specific T cells, leading to T cell activation and the initiation of the adaptive immune response. This process allows the adaptive immune response to focus on specific pathogenic antigens while ignoring self-antigens

Question 2

Define the characteristics of lymphocytes

Answer 2

A type of white blood cell involved in adaptive immune response

Characterised by the presence of a large nucleus and a small amount of cytoplasm.

2 main types:

• B cells = antibody production
• T cells = recognise and eliminate infected or cancerous cells

Question 3

Why do we have lymphocytes?

Answer 3

Important for effective immune response against pathogens and provide long-term protection through the generation of immunological memory - anamnestic response

Able to recognise specific antigens and undergo clonal expansion, leading to the production of large numbers of effector cells that can eliminate the pathogen

Memory lymphocytes can persist for long periods of time and provide rapid protection upon re-exposure to the same pathogen

Question 4

Describe the features of the T cell receptor and the B cell receptor

Answer 4

T Cell Receptor (TCR):

• Structure: The TCR is a protein complex that is present on the surface of T cells. It consists of two different protein chains: an α chain and a β chain or a γ chain and a δ chain. Each chain contains a variable (V) region and a constant (C) region. The V regions of the α and β (or γ and δ) chains form the antigen-binding site of the TCR.
• Antigen Recognition: The TCR recognises antigens in the form of peptide fragments that are presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells (APCs). The TCR specifically interacts with the peptide-MHC complex to initiate T-cell activation.
• Diversity: Like antibodies, TCRs exhibit a high degree of diversity. The TCR genes undergo rearrangement during T cell development, leading to the generation of a diverse repertoire of TCRs capable of recognising a wide range of antigens.
• Co-receptor: TCR signalling is facilitated by a co-receptor known as CD3. CD3 is associated with the TCR complex and helps transmit the intracellular signals triggered by TCR binding to the antigen-MHC complex.

B Cell Receptor (BCR):

• Structure: The BCR is a membrane-bound form of antibody that is present on the surface of B cells. It consists of two identical heavy chains and two identical light chains, each containing variable (V) and constant (C) regions. The V regions of the heavy and light chains form the antigen-binding site of the BCR.
• Antigen Recognition: The BCR recognises antigens directly, without the requirement for antigen presentation by MHC molecules. It can bind to a variety of antigen types, including proteins, carbohydrates, and lipids, through its antigen-binding site.
• Diversity: BCR diversity is generated through a process called V(D)J recombination, which occurs during B cell development. This process involves the rearrangement and recombination of gene segments to generate a diverse repertoire of BCRs capable of recognising different antigens.
• Effector Functions: When the BCR engages with its specific antigen, it can trigger B cell activation and differentiation into antibody-secreting plasma cells. The BCR also functions as an antigen-capturing receptor, internalising antigens for presentation to helper T cells.

Question 5

Explain the concept of lymphocyte repertoire selection

Answer 5

The process by which lymphocytes with specific antigen receptors are selected for survival and proliferation.

Occurs during lymphocyte development in the bone marrow and thymus, where lymphocytes with receptors that recognise self-antigens or are unable to recognise any antigen undergo apoptosis

The surviving lymphocytes then migrate to peripheral lymphoid tissues where they can encounter foreign antigens and mount an immune response

Question 6

How is immunological memory generated and maintained?

Answer 6

Immunological memory is generated and maintained through the clonal expansion and differentiation of memory lymphocytes during the adaptive immune response

Memory lymphocytes are long-lived cells that can persist for ages in the absence of the pathogen

Memory lymphocytes are maintained through homeostatic proliferation, where they divide in the absence of antigen to maintain a pool of memory cells