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 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, of harmful products and own failures within the body, such as the appearance of cells carcinogenic.

Conceiving 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 type of pathology, and almost 150,000 people die every 24 hours around the globe. Can you imagine how these figures would be if we did not have the intricate defense machinery that immunity supposes?

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

• Related article: "Lymphatic system: characteristics, parts and functions"

Overview of the immune system

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

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

For example, tears contain bactericidal substances, the pH of sweat prevents colonization of pathogens and gastric acids limit pathogenic bacterial overgrowth by 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 reach the site of infection, and they respond generally by engulfing bacteria. Macrophages also present antigens to other immune components (components of the microorganism that cause immune responses).

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

• The innate immune system activates the acquired one 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 facing 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 is formed from stem cells in the bone marrow. Its main functions, in general terms, are to adaptively protect the organism from infections and deleterious endogenous processes, such as cancer.

Lymphocytes are a subgroup within leukocytes, a heterogeneous set 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 has about 4,000-11,000 white blood cells per microliter. In any case, this represents only 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 entering fully into the world of T cells, we are interested in sharing a series of data that are common to types B and T alike. Go for it.

Lymphocyte overview

Lymphocytes are agranulocytes, that is, they do not present granules in their cytoplasm, but they do have 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% B cells, and the remaining percentage are null cell bodies.

As we have said, its nucleus is large (it occupies most of the cell), rich in heterochromatin and somewhat misaligned with respect to the cell center. These cell types they have little peripheral cytoplasm, but the scarce that exists houses 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 large cell types. These are the following:

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

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

• You may be interested in: "High Lymphocytes: Causes, Symptoms, and Treatment"

T cell types

It should be noted that there are 4 major types of T cells, although there are some more. We tell you its peculiarities below.

1. Cytotoxic T lymphocytes

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

2. Helper 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 through the secretion of cytokines, proteins directly involved in the effectiveness of the 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, 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 to 7 days to differentiate into effector cells against an unknown antigen, memory cells acquire their function in 1-3 days maximum.

They have a long life, are functionally inactive leukocytes, and can remain circulating in the blood for months and even years.. Vaccines are based on this premise: it is sought that there are both memory T and B lymphocytes activated in a way permanent 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 immune response of the individual, putting an end to the autoimmune reaction and eliminating the T cells that can pose a problem.

These troublesome T cells They are known as self-reactive, that is, they can identify cell molecules essential for our survival as dangerous antigens (like red blood cells), deleting them by mistake. Fortunately, there is a negative selection in the thymus that "gets rid" of these lymphocytes before they can cause damage. When this machinery escapes, regulatory T cells are the final containment machinery.

Resume

The world of T cells (and by extension B cells as well) is extremely complex, as there are so many terms that must be understood and abstract physiological reactions that cannot be observed direct. If we want you to have an idea, this is the following: T cells are an essential part of the acquired immune system, and occur as both memory and effector lymphocytes, which benefit the body in various ways.

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

Bibliographic references:

• Definition and Types of Lymphocytes, and Their Clinical Correlations (Hodgkin Lymphoma), Elsevierconnect.com. Picked up on January 25 in https://www.elsevier.com/es-es/connect/medicina/tipos-de-linfocitos-linfoma-Hodgkin
• Dra, T. P. P. (2012). Panoramic view of the immune system. Las Condes Clinical Medical Journal, 23 (4), 446-457.
• In love Escalona, ​​N. M. (2017). Collaboration between resident and circulating memory CD8 + T cells 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., 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.