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13.4.2. Cellular immune response: T lymphocytes

T lymphocytes: cellular immune response

In addition to the humoral immune response with the action of antibodies on antigens, there is another type of response in which no antibodies are produced. It is the cellular immune response or cell-mediated immune response, which is a very effective process to destroy:

  • Foreign cells from a different individual, even from the same species (for example, transplanted organs).
  • Tumor cells themselves.
  • Cells infected by viruses.
  • Cells that contain intracellularly growing microorganisms (such as tuberculosis bacteria).

The T cells with the help of macrophages, are those which have the cellular immune response. T lymphocytes receive this name because they mature in the thymus.

When a microorganism has entered the living being and is detected, it is phagocytosed  by a macrophage, which digests it through the lysosomes and places some fragments of the antigen (simpler peptides) on its membrane, together with its antigens from themajor histocompatibility complex (MHC-II). This cell is called an antigen-presenting cell.

Afterwards, they will act as cells presenting antigens to a type of T lymphocytes, the helper or helper T lymphocytes (TH), which will recognize  the macrophage MHC antigens as their own (the two cells belong to the same individual), and will recognize the antigen as foreign.

The macrophages  produce cytokines, which enhances the activation and proliferation of T helper, differing in two groups:

  • TH lymphocytes, which release other cytokines that self-activate TH lymphocytes and macrophages. In addition, it causes the differentiation and proliferation of various types of T lymphocytes (cytotoxic and suppressor).
  • TH lymphocytes that activate the transformation of B lymphocytes into antibody-producing plasma cells (humoral immune response).

Here are two types of responses:

  • Cellular immune responseinfected cells can activate  cytotoxic T lymphocytes (T C ) when they recognize the antigen bound to thehistocompatibility complex MHC-I. The T lymphocytes C  bind to the target cell (with foreign antigens in infected cells by viruses or abnormal antigens on cancer cells), and secreted perforin, proteins that perforate the cell membrane, killing the target cell .
  • Humoral immune response :B lymphocytesdifferentiate into antibody-producing plasma cells, which produce theneutralization,opsonization,and activation ofthe complement system.

After having overcome the infection, when the antigen has already been eliminated and it is necessary to stop the immune response, the  suppressor T lymphocytes (TS ) appear, which slow down the response. Sometimes, if the immune response is excessive and dangerous for the body, it attenuates it. In addition, memory cells ( THTC  and B lymphocytes are saved against that specific antigen.

Macrophage function T lymphocyte function

They engulf pathogenic microorganisms.

They are antigen presenting cells.

Macrophage MHC Antigen Recognition.

Binding to antigens for destruction.

They activate B lymphocytes in plasma cells to produce antibodies.

Destruction of infected cells, tumor cells, etc.

A particular type of lymphocytes are the killer cells or NK cells ("natural killer"). These cells, unlike B and T lymphocytes, are larger, have cytoplasmic granules and do not recognize the antigen, so they have a non- specific action. They are responsible for destroying cancer cells or cells infected by viruses.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Aragon. June 2017, option A, question 5.

Define the following concepts related to the immune system: T lymphocytes (0.2 points)

Aragon.  June 2002, option B, part A. Issue 2.-

b) Briefly explain the functions of T lymphocytes (1 mark).

Andalusia, June 2018, alternate, option A, question 5 .

In an individual the thymus is irradiated accidentally, leaving the organ totally dysfunctional.

a) Will your specific immune response be affected? [0.25]

b) Will it be more prone to infections? [0.25]

c) What degree of rejection will this individual have in the face of a transplant? [0.25]

d) And how will it respond to a carcinogenic process? [0.25] Give reasons for the answers.

Oviedo, June 2019, option B, question 5.

James Allison and Tasuku Honjo have been awarded the 2018 Nobel Prize in Medicine for their research in immunotherapy applied to the treatment of tumors. Their research has focused on the self-regulation and inhibition of T lymphocytes. They will go down in history as the "fathers" of immunotherapy.

a. What are lymphocytes? Name the T lymphocytes of the immune system. (Maximum score 0.5)
b. Name the class of immune response in which T lymphocytes participate (Maximum score 0.25)
c. What is the relationship between B lymphocytes and plasma cells? (Maximum score 0.25)

La Rioja, July 2019, option B, question 10

It indicates the main function or functions of cells related to the immune response: macrophagesplasma cellsmemory cells and T lymphocytes. (1 point)

Madrid, July 2019, option B, question 10

a) Define immune response .

b) Indicates the types of specific immune response and explains the action of T lymphocytes in this response.

Basque Country, July 2020, Extraordinary test, question 3A

After the inclusion in the schedule from 1981 of the mumps vaccine (mumps), the incidence rate has been reduced to one tenth, but it seems to have stabilized in the last twenty years in the form of epidemic waves of 4 to 5 years of duration. This year there have been new outbreaks of mumps , especially among university students. According to some researchers, the use during the 1990s of a vaccine that was found to be poorly immunogenic could partially explain this phenomenon.

Answer these questions regarding immunity and immune response:

a) (0.5 points) Define the following immunity terms: congenital , acquired , active, passive and humoral.

b) (0.5 points) Explain what the cellular immune response consists of.

c) (0.5 points) What cells are involved in the cellular immune response?

d) (1 point) Describe the functions of each of these cell types in the cellular immune response.

Madrid, June 2021, question B1

B.1.- In relation to the immune response:

Research on the SARS-CoV-2 Coronavirus infection seems to indicate that cellular immunity may be more important to this virus than in other viral infections.

a) Indicate which cells are involved in cellular immunity and how they act on infected cells (0.5 points).
b) The other type of specific immune response is humoral. Indicate how you can check whether the humoral immune response to this infection has been triggered . Explain why it is more difficult to measure the cellular immune response than the humoral immune response (0.75 points).
c) Indicate three functions of helper T lymphocytes (Th or CD4 +) (0.75 points)

Fundamental insights into the cellular or cell-mediated immune response

The cellular immune response is based on the activity of two types of cells: T lymphocytes and macrophages.

The cellular immune response takes longer to initiate than the humoral immune response, but it is very useful against microorganisms within cells. T lymphocytes destroy these cells, even in the body itself, as infected or tumor cells, which need to be eliminated.

  • The macrophages are phagocytic function and are antigen presenting cells.
  • The T cells bind to antigens and trigger the production of antibodies by B - lymphocytes (or plasma cells). They also destroy infected or tumor cells.