COVID-19 and human immune response
Week after week, numerous articles can be found on coronavirus infection and our related immune response to it.
Antibodies are the effectors of humoral immunity and are therefore present in high concentrations in serum and other biological fluids. The specific humoral response begins when a naive B lymphocyte recognizes its specific antigen in secondary lymphoid tissues and organs.
Initially, B cells secrete IgM isotype in monomeric form and with identical antigen specificity. IgM is the first immunoglobulin synthesized when there is first contact with the antigen. It is mainly secreted as part of the primary humoral immune response for a first and important defense against viral infection.
With respect to the type of antibody secretion, antigens are classified as T-cell dependent or T-cell independent, depending on whether or not they require the assistance of T cells to initiate the antibody response.
Almost all external antigens are T-cell dependent - with the exception of bacterial lipopolysaccharides or polysaccharides - and require an additional signal (cytokine) from specific T-helper lymphocytes that recognize the same antigen for full B cell activation.
In the latter case, the B-cell acts as an antigen presenting cell, presenting the antigen to T-helper cells which, when recognized, allow isotype switching or isotypic commutation process : the change of antibody class. IgM isotype is so going to be converted to a different class of immunoglobulins : IgA, IgE or IgG, depending on the cytokine secreted. This new immunoglobulin will present a higher affinity for the target.
Secondary humoral response to COVID-19
The secondary humoral response induced by subsequent exposure to the same antigen will be characterized by the production of IgG isotype but with higher titer and affinity compared to the first response.
IgG are immunoglobulins primarily involved in the systemic defensive response. They are effective in neutralizing viruses, interacting with the C1 complement fraction and activating the complement cascade with opsonization and phagocytosis of pathogens. In addition, they promote the death of infected cells by antibody-mediated cell-mediated cytotoxicity (ADCC) reactions.
IgA is the predominant class of Ig in secretions (saliva, mucus, tears, colostrum) on mucosal surfaces and prevents penetration and colonization of pathogens.
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