Lymphocytes: What are they and what functions do they carry out in the body?

When we talk about the immune system, we all have a vague notion of its function, which is to protect us. The immune system protects us from any type of threat, not only viruses and bacteria, It is also responsible for eliminating tumor cells, they are the cells that can give rise to a cancer.

Our immune system is something complex and incredibly structured, in reality the immune response is divided into two immune responses. A first generic response, where our body fights all pathogens in a similar way and a specific response, aimed at the specific aggressor. It is in this second type of response where the lymphocytes that we know have their leading role.

In this article we will detail the different responses of the immune system and the specific role of lymphocytes in the fight against the aggressors.

• Related article: "Main Cell Types of the Human Body"

What are lymphocytes?

In our day to day we are exposed to many types of external aggression. These attacks can be physical (heat, trauma, radiation), biological (viruses, bacteria and parasites), or chemical (poisons, alcohol, etc.).

Our immune system is responsible for maintaining the homeostasis of our body, this means that it has to maintain our body in a constant balance against the aggressions and changes of the external environment. But, to achieve this, he has to fight against those responsible and eliminate them. Many organs, mechanisms and systems collaborate with each other to defend us from daily threats.

Lymphocytes are a type of cell in our immune system., whose main role is to identify and eliminate pathogens present in our body.

A pathogen is any microorganism that can cause disease. Lymphocytes can then fight against any threat, it is important to note that they can protect us from cells that have stopped working properly, such as tumor cells.

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The nonspecific immune response

When a pathogen enters our body, it triggers an immune response. In a very simplified way, some resident cells in our tissues, the macrophages, are the first to identify and fight the pathogen. To eliminate it, they are going to surround it using their cytoplasmic membrane and introduce it into their cell interior to break it down into small parts. This process, in which we can say that the macrophage engulfs the pathogen, is known as phagocytosis.

The macrophages will also generate chemicals to warn other immune cells of the offender's presence. When, for example, we cut ourselves, not only one germ enters our body, but millions of them. In a kiss, for example, 80 million bacteria can be exchanged. These data provide an idea of ​​the number of cells involved in the immune response.

The chemicals released by the macrophages alert other cells circulating in the bloodstream, mostly neutrophils. These are not resident cells of tissues, such as macrophages. Neutrophils are able to leave the bloodstream and migrate to tissues, this mechanism is known as diapedesis. Once in the tissues, the neutrophils also have phagocytosis capabilities to combat the aggressors.

Macrophages and neutrophils can recognize the aggressors because they express, on their cellular exterior, a large number of receptors. These membrane-anchored proteins are capable of detecting and recognizing different molecules that belong to the aggressors. For example, they can distinguish peptidoglycan, which is the polymer that makes up the cell wall of bacteria.

This first part of the immune response is known as the nonspecific response or innate immunity. depends on the pathogen, nor does it need sensitization, that is, having been exposed to the pathogen with anteriority.

• Related article: "The 4 types of pathogens (and their characteristics)"

NK cells and the nonspecific immune response

Within the nonspecific immune response, a lesser-known type of lymphocytes also participate. Until now we have seen how our body deals with a microorganism (bacteria, viruses, fungi, protozoa and prions). However, NK (Natural Killer) cells have the mission of destroying altered cells: tumor cells or infected cells.

This type of innate immunity cells do not use, like the others, phagocytosis to destroy malignant cells.. The process with which they eliminate malignant cells is not very clear, but it is known that it is carried out through the secretion of small proteins that destroy the target cell.

If the innate immune response is not sufficient, the immune system has a type of cell that serve to combat the specific aggressor and are, therefore, more effective. Do you know which cells are about? Yes, the lymphocytes.

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Characteristics of lymphocytes

Lymphocytes, like macrophages (mature monocytes) and neutrophils, along with eosinophils and basophils, are part of the so-called white blood cells (leukocytes). They are the blood cells responsible for executing the immune response.

Lymphocytes are the smallest of the white blood cells, approximately 10 to 16 micrometers, and represent between 20% and 40% of white blood cells in the bloodstream (which are those that are measured in a analysis).

We can distinguish three types of lymphocytes: B cells, T cells, and NK cells (which we have already introduced). These will act together in the immune response.

Synthesis and maturation of lymphocytes

All lymphocytes originate in the bone marrow. Later, in their maturation process, they migrate to the primary lymphoid organs, these are the bone marrow and thymus. T lymphocytes mature and differentiate in the thymus and B lymphocytes in the bone marrow. B lymphocytes mature in two stages, first in the bone marrow and then in the secondary lymphatic organs, where they are exposed to antigens.

It is interesting to understand how this maturation process occurs in order to understand how the specific immune response occurs.

During the maturation process, B lymphocytes gradually acquire different molecules on their membrane, among them, the receptor for its specific antigen. We already said, when we talked about non-specific immunity, that macrophages and neutrophils are capable of detecting different molecules that are common to many pathogens. However, the lymphocyte receptor is specific for a particular antigen.

After its maturation, are grouped in the lymph nodes, spleen, and lymphoid tissues, which are responsible for cleaning the blood. Thus, via the blood or the lymph, it is how the B lymphocytes come into contact with the antigens for which they are specific. This phenomenon is called activation.

Roles of B and T lymphocytes in the adaptive immune response

When they come into contact with the antigen, the B lymphocytes differentiate into two types of cells, one that act immediately and others that function as a reserve: plasma cells and cells of memory.

plasma cells

Plasma cells are the cells responsible for executing the specific response. They use antibodies, which are small, slingshot-shaped proteins capable of surrounding the specific microorganism. In this way, other cells, with the capacity to destroy, can locate and eliminate the previously surrounded pathogen.

This specificity is given because the antibodies are only capable of binding to the antigens that are expressed on the membrane of the specific pathogen. It is as if antibodies can only bind to humans through our hands, monkeys have hands too, but antibodies can only bind to the hands of Homo Sapiens.

memory cells

As their name indicates, the function of memory cells is save the information about the microorganism and remember this. In this way, the moment we are exposed to the same pathogen again, all the cells of memory (previously activated) will be able to act immediately, secreting an enormous amount of antibodies. This mechanism is what is called immune memory. If our body already has. memory cells, we will be able to eliminate the pathogen more quickly and effectively, this is the principle of vaccines.

T lymphocytes

There are two main types of T lymphocytes, CD8+ cytotoxic T lymphocytes and helper T lymphocytes (CD4+).

CD8+ cytotoxic T lymphocytes

CD8+ cytotoxic T lymphocytes are involved in cellular immunity. This means that are capable of recognizing and destroying cells infected by intracellular pathogens. They do this by injecting proteins, which are toxic to all cells in the human body, into the infected cells.

Helper T lymphocytes (CD4+)

helper T cells cannot destroy infected cells or microorganisms. These lymphocytes have a regulatory role, they activate other cells such as B lymphocytes. The activation process is particular and deserves to be detailed.

In addition to their engulfing function, macrophages are also antigen-presenting cells. The macrophages not only engulf the pathogen, but also place bits of it (proteins) on its surface, as if they were a kind of pennants. These proteins, which are placed on the membrane of macrophages, are what we know as antigens, a molecule that the body recognizes as foreign.

Macrophages present antigens to helper T lymphocytes or TCD4+. These will activate the B lymphocytes, thus amplifying the response of the immune system. We already know that the more antibodies, the greater the response.