Explain why the third line of defence takes more time to respond to an infection than the second line of defence.

The third line of defence, also known as the adaptive immune response, takes more time to respond to an infection than the second line of defence, or the innate immune response, due to several reasons:

1. Specificity: The third line of defence is highly specific. It targets specific pathogens that the body has previously encountered. This specificity requires time to identify the exact pathogen and produce the specific antibodies or cells to fight it.

2. Clonal Selection and Expansion: Once the specific pathogen is identified, the specific lymphocyte (B-cell or T-cell) that can fight this pathogen undergoes clonal selection and expansion. This process involves the proliferation and differentiation of the lymphocyte into a large number of effector cells and memory cells. This process takes time.

3. Production of Antibodies: In the case of B-cells, once they are activated, they differentiate into plasma cells that produce antibodies. The production and release of these antibodies into the bloodstream also takes time.

4. Memory Response: One of the key features of the third line of defence is the development of immunological memory. Some of the activated lymphocytes differentiate into memory cells that provide long-term immunity against the specific pathogen. The development of these memory cells and the memory response also takes time.

In contrast, the second line of defence is non-specific and acts quickly to a wide range of pathogens. It includes physical and chemical barriers like skin and mucous membranes, and internal defences like phagocytes, natural killer cells, inflammation, and fever. These responses are immediate and do not require time for identification of the pathogen or production of specific antibodies.