Third line of defence
The third line of defence is the adaptive immune response, which provides a highly specific and long-lasting defence against pathogens. Unlike the innate immune response, which is generalised, the adaptive immune system recognises and targets specific antigens. This system involves specialised immune cells called lymphocytes, including B cells and T cells, which are responsible for the humoral and cell-mediated immune responses.
Humoral Response (B Lymphocytes and Antibodies)
The humoral response is primarily mediated by B lymphocytes, or B cells. These cells recognise and respond to specific antigens present on pathogens such as bacteria, viruses or toxins.
Recognition and Activation: B cells have surface receptors that are specific to particular antigens. When a B cell encounters an antigen that matches its receptor, it binds to the antigen, initiating its activation. This interaction often requires additional signals from helper T cells, which assist in fully activating the B cells.
Differentiation and Antibody Production: Once activated, B cells proliferate and differentiate into plasma cells. These plasma cells are responsible for producing large quantities of antibodies, also known as immunoglobulins. Each antibody is specific to the antigen that triggered the response. The antibodies circulate through the blood and lymphatic system, binding to the pathogens with high specificity.
Pathogen Neutralisation and Marking for Destruction: Antibodies neutralise pathogens in several ways. They can directly neutralise toxins or viruses, preventing them from infecting cells. Additionally, antibodies can tag pathogens for destruction by other immune cells through a process called opsonisation. This tagging enhances the ability of phagocytes, such as macrophages and neutrophils, to recognise and engulf the pathogens.
Memory B Cells: In addition to producing antibodies, some activated B cells become memory B cells. These cells persist in the body for years or even decades. Memory B cells are capable of recognising the same antigen if it re-enters the body. This rapid response upon re-exposure is the basis for long-term immunity and the principle behind effective vaccination strategies.
Cell-Mediated Response (T Lymphocytes)
The cell-mediated response involves T lymphocytes, or T cells. Unlike B cells, T cells do not produce antibodies but instead directly target and eliminate infected or abnormal cells.
Helper T Cells (CD4+ T Cells): Helper T cells recognise antigens presented by other immune cells through Major Histocompatibility Complex (MHC) class II molecules. Upon activation, helper T cells release cytokines that stimulate various immune cells, including B cells, cytotoxic T cells and macrophages. This enhances the overall immune response.
Cytotoxic T Cells (CD8+ T Cells): Cytotoxic T cells are specialised in directly killing infected or cancerous cells. They recognise antigens presented by MHC class I molecules on the surface of target cells. Upon recognition, cytotoxic T cells release toxic substances such as perforin and granzymes, which induce apoptosis (programmed cell death) in the infected or abnormal cells.
Regulatory T Cells (Tregs): Regulatory T cells help maintain immune system balance by preventing excessive immune responses and autoimmune reactions. They suppress the activity of other immune cells to ensure that the immune response is proportionate and does not target the body’s own tissues.
Memory T Cells: After the pathogen is cleared from the body, a subset of activated T cells differentiates into memory T cells. These cells do not participate in the immediate response but instead persist in the body for extended periods, sometimes for years or even decades.
