Question

A person gets infected after breathing in air-borne droplets containing SARS-CoV-2. Outline step-by-step the immune defense at different anatomical levels of the lymphatic system from first encounter to the time when the patient develops serum anti-viral antibodies.

Answer 1

After breathing in air-borne droplets containing SARS-CoV-2, the initial protective factors are the components of the innate immunity. The normal bacterial flora of the nose and oral cavities, the lysozyme present in tears are all protective factors. Once it reaches the respiratory tract, the pulmonary alveolar macrophages helps by phagocytosis of the virus. Now, if these defence mechanisms fail, the virus enters the blood stream. The virus is identified foreign by the cellular elements of the innate immunity and this leads to activation of inflammation by cytokines, phagocytosis of the virus by the macrophages, dendritic cells and activation of the complement pathway.

If the innate immune response described above was not able to provide adequate defence against the virus, the adaptive immune response comes into play. Here, the virus is presented to the T lymphocytes by the antigen presenting cells.

When the antigen is presented by the antigen presenting cells to the CD8+ T cells with the help of major histocompatibility complex - MHC I, this leads to activation of the CD8+ cytotoxic T cells which mediates inflammation and cause cellular killing to destruct the organism. This is cell-mediated immunity.

When the antigen is presented by the antigen presenting cells to the CD4+ T lymphocytes with the help of major histocompatibility complex - MHC II, this activates the CD4+ helper T cells. This now mediates inflammation by recruiting inflammatory cells. The activated T helper cells inturn activates the B lymphocytes. the B lymphocytes now differentiate into  plasma cells which produces antibodies, leading to destruction of the virus.