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 the major 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 response:
infected cells can activate cytotoxic T lymphocytes (TC) 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 the neutralization, opsonization, and activation of the 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 (TH, TC and B lymphocytes are saved against that specific antigen.
| Macrophage function | T lymphocyte function |
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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.