explain elaborately with scientific knowledge on the entire defensive procedure when our body gets inaded by covid the second time

When the body is invaded by the SARS-CoV-2 virus (which causes COVID-19) for the second time, the immune system responds in a more robust and rapid manner due to a process called immune memory. Here's a step-by-step explanation of this process:

1. Recognition of the Virus: The immune system recognizes the virus as a foreign invader. This is done by proteins on immune cells called receptors, which can identify specific structures on the virus called antigens.

2. Activation of the Immune Response: Once the virus is recognized, the immune response is activated. This involves a variety of cells and molecules. The first line of defense is the innate immune response, which includes physical barriers like the skin and mucous membranes, as well as immune cells like macrophages and neutrophils that can engulf and destroy the virus.

3. Adaptive Immune Response: If the virus manages to evade the innate immune response, the adaptive immune response is activated. This is a more specific and targeted response, involving cells called T and B lymphocytes. T cells can directly kill virus-infected cells, while B cells produce antibodies that can neutralize the virus.

4. Immune Memory: The first time the body is infected with SARS-CoV-2, the immune system has to learn to recognize and fight the virus. This process takes time, which is why people can get sick. However, during this process, some T and B cells become memory cells. These cells remember the virus, so if it invades the body again, they can respond more quickly and robustly. This is the basis of immunity.

5. Second Infection: If the body is invaded by SARS-CoV-2 a second time, the memory cells recognize the virus and activate the immune response much more quickly. This can often prevent the person from getting sick, or at least reduce the severity of the illness. This is why people who have recovered from COVID-19 are usually at lower risk of getting it again.

6. Variants and Immune Evasion: However, viruses can mutate and change their antigens, which can allow them to evade the immune response. This is why new variants of SARS-CoV-2 can sometimes cause reinfections. However, the immune system is very adaptable and can often learn to recognize and fight new variants as well.

It's important to note that this is a simplified explanation, and the actual process is much more complex and involves many more components of the immune system. Also, individual immune responses can vary greatly, and are influenced by many factors, including age, overall health, and the presence of other medical conditions.