T cells: what they are and how they work in the immune system

The human immune system is defined as a complex network of cells, tissues, organs (and the substances they produce) that help the body fight infections and other diseases. We are not just talking about viruses and bacteria: this machinery protects us from the external elements of allergies, harmful products and from our own failures in the body, such as the appearance of cancer cells.

Conceiving the existence of life as we know it today without the immune system would be impossible. At any given time, it is estimated that 95% of humans on Earth suffer from some form of pathology, and around 150,000 people die every 24 hours worldwide. Can you imagine what those numbers would be if we didn’t have the complex defense mechanism of immunity?

Today we come to show one cell types essential to understand acquired immunity in humans: T cells or T lymphocytes. If you want to know all about them, keep reading.

    General information on the immune system

    Talking about T cells without laying some foundation first is like starting to build a house on the roof. Therefore, in the following lines, we give you some generalities of great interest about the immune system.

    Historically, it has been considered that we have two types of immunity: innate and acquired. The first is the one we were born with, and which generally responds to the possible entry of pathogens into the body. In the innate immune system, we do not only find cell bodies, because mucous membranes, epithelia, skin, secretions such as sweat or saliva and the same cough reflex are also part of it.

    For example, tears contain bactericidal substances, the pH of sweat prevents colonization of pathogens, and stomach acids minimize pathogenic bacterial proliferation. Biological barriers are based on physical, chemical and biological components (the intestinal flora, for example, prevents the growth of other bacteria).

    The most well-known cell bodies of the innate immune system are neutrophils and macrophages.. They are the first to reach the site of infection and usually respond by phagocytizing bacteria. Most macrophages present antigens to other immune components (components of the microorganism that cause an immune response).

    On the other hand, we have acquired immunity, which improves as the individual is exposed to potential pathogens. We will discuss this in more detail in the following lines, but you should keep the following in mind:

    • The innate immune system activates acquired knowledge in response to infections.
    • The acquired immune system uses the effector organisms of innate immunity to eliminate pathogenic microorganisms.

    For both reasons, distinguishing between these types of systems is useful in terms of awareness, but not so much from a physiological point of view. We are faced with two sides of the same coin with a common goal: the short and long term protection of the individual.

    What are T cells?

    T cells or T lymphocytes are defined as a type of white blood cell that forms from stem cells in the bone marrow. Its main functions, in general, are to adaptively protect the body against infections and harmful endogenous processes, such as cancer.

    Lymphocytes are a subgroup of leukocytes, a heterogeneous collection of blood cells that do not contain typical pigments, which is why they are known as white blood cells. It is generally estimated that a healthy human being has about 4,000 to 11,000 white blood cells per microliter. However, this is only 1% of the total blood volume, as the red blood cells far exceed them.

    T cells occur in the bone marrow, migrate to the scaffold (where they mature), and differentiate into other subtypes.. Before we dive fully into the world of T cells, we find it interesting to share a number of data common to B and T types. Let’s do this.

    General lymphocytes

    Lymphocytes are agranulocytes, i.e. they do not have granules in their cytoplasm, but a large compartmentalized nucleus yes. They are around 8-10 microns in diameter on average (although there are increasingly smaller ones) and their shape is rounded. There are 3 types of lymphocytes: B, T and null cells. Usually 80% of the circulating lymphocytes are T cells, 15% are B cells and the remaining percentage are null cell bodies.

    As we have said, its nucleus is large (occupies most of the cell), rich in heterochromatin and with a somewhat inappropriate placement relative to the center of the cell. These cell types they have little peripheral cytoplasm, but the few that do house it harbor mitochondria, a small Golgi apparatus, and a few rough endoplasmic reticulum (RER) profiles. It also has a high number of ribosomes and some lysosomes.

    In lymphocytes (whether B or T), there are two main types of cells. These are:

    • Memory cells: do not participate directly in the immune response, but remain in circulation and divide as an already known pathogen enters the system.

    • Effector cells: are immunocompetent lymphocytes capable of carrying out the immune response itself. It is they who eliminate pathogens, as well as other cell bodies.

    • You might be interested in: “Elevated Lymphocytes: Causes, Symptoms and Treatment”

    Types of T cells

    It should be noted that there are 4 main types of T cells, although there are others. We explain its peculiarities below.

    1. Cytotoxic T lymphocytes

    Also known as CD8 + (due to the nature of its membrane surface molecules), cytotoxic T cells detect peptides presented by MHC class I molecules and destroy infected cells. Translated into nicer language, this feature could be defined in which they recognize infected or transformed cells and destroy them by mechanisms that cause apoptosis, i.e. their destruction.

    2. Cooperating T lymphocytes

    Also known as CD4 + cells, these cell bodies are responsible for initiating the immune response cascade. Its function is activate the functionality and action of other immune cells by secreting cytokines, proteins directly involved in the efficiency of the response.

    3. Memory T lymphocytes

    What defines this type of T cell is its ability to remain at rest after removal of the antigen (from the pathogenic bacteria). Thanks to them, a more efficient response to the same antigen is allowed in future exposures, compared to the response power provided by virgins. While a virgin T cell takes 5-7 days to differentiate into effector cells against an unknown antigen, memory cells take on their function within 1-3 days.

    They have a long lifespan, are functionally inactive leukocytes, and can remain circulating in the blood for months or even years.. The vaccines are based on this premise: we seek to have both T and B memory lymphocytes permanently activated in the individual, in order to generate immunity against the pathogen that has been injected into the patient.

    Regulatory T lymphocytes

    As the name suggests, they are responsible for regulating an individual’s immune response, stopping the autoimmune reaction, and eliminating T cells that may be causing problems.

    These problematic T cells they are called self-reactive, which means that they can be identified as dangerous antigens for cellular molecules essential to our survival. (Like globules red), deleting them by mistake. Fortunately, there is negative selection in the scam that “gets rid” of these lymphocytes before they can do any harm. When this machinery escapes, regulatory T cells are the final containment machinery.

    summary

    The world of T cells (and by extension B cells as well) is extremely complex, as there are so many terms that need to be understood and abstract physiological reactions that cannot be directly observed. If we want you to stick with one idea, it’s this Next: T cells are an essential part of the acquired immune system and occur as both memory lymphocytes and effectors, Which benefit the body in different ways.

    Antigens are one of the microorganisms that cause an immune response in us. When one of them enters our body, cells recognize it and activate different mechanisms. Thanks to memory cells, the more we expose ourselves to a specific antigen, the more we will develop tolerance to it over the long term.

    Bibliographical references:

    • Definition and types of lymphocytes, and their clinical correlations (Hodgkin lymphoma), Elsevierconnect.com. Collected January 25 from https://www.elsevier.com/es-es/connect/medicina/tipos-de-linfocitos-linfoma-Hodgkin
    • Dra, TPP (2012). Panoramic view of the immune system. Revista Mèdica Clínica Les Comtes, 23 (4), 446-457.
    • Enamorat Escalona, ​​New Mexico (2017). Collaboration between resident and circulating CD8 + memory T lymphocytes improves antitumor immunity.
    • Font Pons, M. (2020). Automatic classification of abnormal lymphocytes from low prevalence lymphomas using a few stroke learning (undergraduate thesis, Universitat Politècnica de Catalunya)
    • Martín, DD, Rodríguez, HB, Cort, MB and Soto, M. Á. M. (2013). T cell effector functions. Accredited Continuing Medical Education Program in Medicine, 11 (28), 1742-1751.
    • Muñoz Petecua, North Carolina (2011). Phenotypic and functional characteristics of CD8 + memory T lymphocytes.

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