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    B and T-cell mediated immunity


    There are two main mechanisms of immunity within the adaptive immune system – humoral b lymphocytes cell mediated immunity and cellular (t lymphocytes cell mediated immunity).

    Humoral immunity and cell-mediated immunity are two types of an adaptive immune response that allow the human body to defend itself in a targeted way against pathogens such as bacteria, viruses and toxins. 

    There is a relation between these two types of immune responses because both rely on the functions of lymphoid cells but there are also some important differences.

    Humoral immunity to a specific infection or disease can be acquired by, for example, giving antibodies from someone previously exposed to the same infection. However, cell-mediated immunity involves a set of molecular components and processes that differ from antibody-mediated immunity.

    In this article, we will describe the main concepts to understand T-cell- mediated and B-cell-mediated immunity to any pathogen.

    What is B mediated immunity?

    B cells (which are also called B lymphocytes) are a subtype o lymphocyte; these cells are originated in the bone marrows and they circulate via the blood stream. As we have addressed in previous articles these lymphocytes play a key role in the adaptive immune response to Covid-19.

    Humoral immunity is also referred to as antibody-mediated immunity. With the help of helper T lymphocytes, B lymphocytes derive in plasma B lymphocytes that can produce antibodies against a specific antigen. The humoral immune system addresses the antigens of freely circulating pathogens, or outside infected cells.

    Antibodies role in humoral immunity

    These antibodies that are produced by plasma B cells can bind with the antigens and fight against them in order to prevent them to completely enter into the host cells.

    In this sense, when an unfamiliar substance (antigen) enters the body, the immune system recognizes it as unknown and actives some mechanisms, including antibody production, in order to fight against the antigen.

    Therefore, antibodies (immunoglobulins) are protective proteins that our immune system produces in response to the presence of an antigen and protects us from them.

    This is the main reason why B cells are so important in this process because each B cell produces antibodies which are specifically created to fight against that pathogen; therefore, they can bind to many different target antigens to prevent from many different types of infections.

    On the other hand, cell mediated immunity is a type of adaptive immune response that does not produce antibodies, in this type of immunity T-lymphocytes, NK (Natural Killer cells) and macrophages are activated.  This cell-mediated immunity plays an important role in controlling viral, chlamydia, rickettsia and protozoan infections such as trypanosomes as antibodies cannot penetrate and attack intracellular pathogens which multiply within the host cells.


    Immunity mediated by B cells and antibodies

    Firstly, we should start answering the question what are antibodies? After all the process we have described before, once these antibodies are in the blood stream and lymphatic system, we can say these are ready to function as defensive molecules with direct and indirect immune functions. Between these immune functions we could underline:

    The neutralization of infectious agents: they do it by blocking or by antibody-dependent cellular cytotoxicity.

    Activation of the complement system. The complement system is part of the innate immunity, which increases the ability of antibodies and lymphocytes to remove pathogens and infected cells from the body.

    Binding of foreign substances to be destroyed: by opsonization and phagocytosis.

    In this sense, antibodies neutralize antigens primarily through mechanisms of attachment and accumulation: the aggregation of neutralizing antibodies on the matching viral particles with the antigen would inhibit the ability of this virus to infect to other cells.

    Furthermore, antibodies can also take part in the process that lead to lysis or killing of infectious cells by the activation of the complement cascade or interaction with effector cells and release of cytokines.

    You can discover everything about Humoral and antibodies immunity to SARS-CoV-2 in this article: learn more.

    What is T cell mediated immunity?

    As we have mentioned before, T cells are responsible for cell mediated immunity.  This T cells are a type of lymphocytes that are characterized by expressing a special receptor on the surface of the membrane, the T- cell receptor (also called TCR). T cells are produced in the bone marrow and mature in the thymus,. In this process we can differentiate two major T cell subsets:

    CD4+ Helper T cells: As the name suggests helper T cells ‘help’ other cells of the immune system. Moreover, helper T cells primarily recognize antigens and secreting cytokines for activating both cytotoxic T cells and B cells.

    CD8+ Cytotoxic T cells: cytotoxic T cells recognize and kill virally infected cells and tumors, actually they destroy the infected cells by apoptosis (discover what is apoptosis in this article: see more).

    Furthermore, CD4+ regulatory T cells are third subset of T cells, whose function is to inhibit the immune response, modulating  the immune system in such a way to tolerate the self-antigens, preventing autoimmune diseases.

    Main differences between B cells and T cell-mediated cells

    The main difference between T cells and B cells is that, unlike the antibodies, the T cells, through the TCR, are not able to bind the antigen directly, instead of that, the need  to antigen processing, consisting in degradation of protein antigen to peptides..  We describe bellow more differences between this two types of lymphocytes:

    • T cells are involved in the cell-mediated immunity and B cells are involved in humoral or antibody-mediated immunity.
    • T cells secrete lyctokines and B cells Secrete antibodies.
    • T cells defend against pathogens including viruses, protists and fungi that enter the cells in the body, and B cells fight against bacteria, and viruses in the bloodstream or lymph.
    • T cells develop from bone marrow stem cells and mature in the thymus while B cells mature in the bone marrow and complete their maturation in secondary lymphoid organs.
    • T cells don’t have surface antigens receptors and B cells do, thus T cells do not directly recognize antigens. Instead, they recognize by TCR antigens peptides presented on major histocompatibility complexes ( MHCs ) that cells use to display which proteins are inside of them.
    • T cells recognize viral antigens on the outside of the infected cells while B cells recognize antigen on the surface of the bacteria and viruses.
    • There are 3 types of T Cells: Helper T cells, cytotoxic T cells, and regulatory T cells and there are two types of B cells: plasma cells and memory cells.
    • T cells move to the site of infection while B cells do not move to the site of infection
    • T cells have an inhibitory effect on the immune system, but B cells don’t.

    Humoral and cellular immunity: antibodies and cells

    In conclusion, T-lymphocytes and B-lymphocytes are two types of lymphocytes that initiate an immune response against external substances in the organism.

    T-helper cells trigger the production of antibodies by plasma cells. Cytotoxic T cells kill pathogens by inducing apoptosis. B cells express specific antibodies against different antigens by recognizing them in the circulatory system. Which lead us to the conclusion that the main difference between T cells and B cells is their role in the immune response and the method of antigen recognition.

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