T cells: what are they and how do they function in the immune system?

Let's take a look at what T cells are and how they interact with the other elements of the body.

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 only talking about viruses and bacteria: this machinery defends us from external elements in allergies, from harmful products and from our own failures within the organism, such as the appearance of cancer cells.

To conceive the existence of life as we know it today without the immune system would be impossible. At any given time, it is already estimated that 95% of human beings on Earth have some kind of pathology, and almost 150,000 people die every 24 hours around the globe. Can you imagine what these figures would be like if we didn't have the intricate defense machinery of immunity?

Today we come to show you one of the cell types cell types essential to understanding human acquired immunity: the T cells or T lymphocytes.. If you want to know all about them, read on.

General information about the immune system

Talking about T lymphocytes without first laying certain foundations is like starting to build a house from the roof up. Therefore, in the following lines we will tell you some very interesting generalities about the immune system.

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

For example, tears contain bactericidal substances, the pH of sweat prevents the colonization of pathogens and gastric acids limit pathogenic bacterial overgrowth to a minimum. Biological barriers are based on physical, chemical and biological components (the intestinal flora, for example, prevents other bacteria from growing).

The most famous cell bodies of the innate immune system are neutrophils and macrophages.. They are the first to arrive at the site of infection, and respond in a general way by phagocytizing bacteria. Macrophages also present antigens to other immune components (components of the microorganism that cause immune response).

On the other hand, we have acquired immunity, which is perfected the more the individual is exposed to potential pathogens. We will talk about it in more detail in the following lines, but you should keep in mind the following:

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

For both reasons, making a distinction between these types of system is useful at an informative level, but not so much from a physiological point of view. We are dealing with two sides of the same coin with a common purpose: 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 is formed from stem cells in the bone marrow. Their main functions, broadly speaking, are to protect the organism adaptively from infections and deleterious endogenous processes, such as cancer.

Lymphocytes are a subgroup of leukocytes, a heterogeneous group of blood cells that do not contain typical pigments, hence they are known as white blood cells. In general, it is estimated that a healthy human being has about 4000-11 000 white blood cells per microliter. However, this is only about 1% of the total blood volume, as red blood cells greatly exceed them.

T-lymphocytes are produced in the bone marrow, migrate to the thymus (where they mature) and differentiate into other subtypes. Before we dive right into the world of T cells, we find it interesting to share a number of facts that are common to B and T types alike. Let's get to it.

General information about lymphocytes

Lymphocytes are agranulocytes, ie, no granules in their cytoplasm, but a large compartmentalized nucleus. They are about 8-10 micrometers in diameter on average (although there are larger and smaller ones) and their shape is rounded. There are 3 types of lymphocytes: B, T and null cells. Generally 80% of circulating lymphocytes are T cells, 15% are B cells and the remaining percentage are null cell bodies.

As mentioned above, their nuclei are large (occupying most of the cell), rich in heterochromatin and somewhat misaligned with respect to the cell center. These cell types have little peripheral cytoplasm, but what little there is harbors mitochondria, a small Golgi apparatus and a few rough endoplasmic reticulum (RER) profiles.. It also has a high number of ribosomes and a few lysosomes.

Within lymphocytes (either B or T), there are two major cell types. These are the following:

• Memory cells: they do not participate directly in the immune response, but remain circulating and divide as soon as a known pathogen enters the system.

• Effector cells: these are the immunocompetent lymphocytes capable of carrying out the actual immune response. They are the ones that eliminate pathogens, together with other cell bodies.

• Types of T cells

  It should be noted that there are 4 main types of T cells, although there are some more. We will tell you about their particularities below.

  1. Cytotoxic T lymphocytes

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

  2. T-helper T lymphocytes

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

  3. Memory T lymphocytes

  What defines this type of T cells is their ability to remain quiescent after the elimination of the antigen (of the pathogenic bacteria). Thanks to them, they allow for a more effective response to the same antigen in future exposures, compared to the potency of response provided by virgins.. While a virgin T cell takes 5-7 days to differentiate into effector cells against an unknown antigen, memory cells acquire their function in 1-3 days maximum.

  They are long-lived, functionally inactive leukocytes, and can remain circulating in the blood for months or even years.. Vaccines are based on this premise: the aim is to have both memory T and B lymphocytes permanently activated in the individual, in order to generate immunity to the pathogen that has been injected into the patient.

  Regulatory T lymphocytes

  As their name suggests, they are responsible for regulating the individual's immune response, putting an end to the autoimmune reaction and eliminating T cells that may pose a problem.

  These problematic T cells are known to be self-reactive, i.e. they can identify as dangerous antigens molecules of cells essential for our survival (such as red blood cells), eliminating them by mistake. (such as red blood cells), eliminating them by mistake. Fortunately, there is negative selection in the thymus that "gets rid" of these lymphocytes before they can cause harm. When they escape this machinery, 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 many, many terms that must be understood and abstract physiological reactions that cannot be observed directly. If we want you to stay with one idea, it is this: T cells are an essential part of the acquired immune system, and come in the form of both memory and effector lymphocytes, which benefit the immune system in various ways.which benefit the organism in various ways.

  Antigens are parts of microorganisms that elicit 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 are exposed to a particular antigen, the more tolerance we develop to it in the long term.

  Bibliographic references:
  • Definition and types of lymphocytes, and their clinical correlates (Hodgkin's lymphoma), Elsevierconnect.com. Retrieved January 25 from https://www.elsevier.com/es-es/connect/medicina/tipos-de-linfocitos-linfoma-Hodgkin.
  • Dr, T. P. P. P. (2012). Panoramic view of the immune system. Revista Médica Clínica Las Condes, 23(4), 446-457.
  • Enamorado Escalona, N. M. (2017). Collaboration between resident and circulating memory CD8+ CD8+ T lymphocytes enhances antitumor immunity.
  • Font Pons, M. (2020). Automatic classification of abnormal lymphocytes from low-prevalence lymphomas using few-shot learning (Bachelor's thesis, Universitat Politècnica de Catalunya).
  • Martín, D. D. D., Rodríguez, H. B., Corte, M. B., & Soto, M. Á. M. (2013). Effector functions of T lymphocytes. Medicine-Accredited Continuing Medical Education Program, 11(28), 1742-1751.
  • Muñoz Petecua, N. C. (2011). Phenotypic and functional characteristics of memory CD8+ T lymphocytes.