Learning Goals: Adaptive Immunity and the Lymphatic System
- Describe the role of the lymphatic system in immunity.
- Explain how B and T lymphocytes act in specific immune responses.
Introduction
The adaptive immune system is the body’s specialised defence against specific pathogens. Unlike the innate immune system, it can “remember” previous encounters, enabling a faster and stronger response to future infections. It works closely with the lymphatic system, which not only transports immune cells and antigens but also acts as a key site for immune activation.
Key Concepts
Lymphatic System
- Composed of lymph (fluid containing immune cells), lymphatic vessels, and lymph nodes.
- Transports immune cells (B and T lymphocytes) and antigens throughout the body.
- Lymph nodes filter lymph and serve as meeting points where B and T cells encounter antigens, often presented by antigen-presenting cells (APCs) such as dendritic cells and macrophages.
The lymphatic system includes primary lymphoid organs (bone marrow and thymus), where lymphocytes develop and mature, and secondary lymphoid organs (lymph nodes, spleen, tonsils), where immune responses are activated. It transports lymph, immune cells, and antigens throughout the body, playing a central role in immunity.
Humoral Immunity – B Lymphocytes (B Cells)
- Mature in the bone marrow.
- On activation, differentiate into:
- Plasma cells – secrete antibodies specific to the pathogen’s antigens.
- Memory B cells – long-lived cells providing immunity against future infections by the same pathogen.
- Antibodies work by:
- Neutralising pathogens or toxins.
- Opsonising pathogens to enhance phagocytosis.
- Activating the complement system to help destroy microbes.
Cell-Mediated Immunity
Helper T Cells (Th Cells)
- Activated when their T cell receptor recognises antigens presented on MHC Class II molecules by APCs.
- Release cytokines that stimulate:
- B cells to produce antibodies (linking to humoral immunity).
- Cytotoxic T cells to kill infected cells.
Cytotoxic T Cells (Tc Cells)
- Recognise antigens displayed on MHC Class I molecules (found on almost all nucleated cells).
- Kill infected or abnormal cells by releasing perforin and granzymes, triggering apoptosis (programmed cell death).
Clonal Selection
- When a lymphocyte recognises its specific antigen, it is “selected” to rapidly divide and produce many identical cells (clones), all targeting the same antigen.
Memory Cells
- B and T memory cells can remain in circulation for years (sometimes decades).
- Provide a faster, more effective secondary response if the pathogen is encountered again.