Adaptive Immune Response: Cell-Mediated Response

Article 7

T-Cell Overview

The adaptive immune response can be divided into humoral and cell-mediated immunity. Here we will discuss cell-mediated immunity. The primary cell in this type of immunity is the T cell. T cells have an antigen-binding, Ig-like surface receptor—the T-cell receptor (TCR). Genes that encode the TCR, like Ig genes, can be rearranged, resulting in specificity and affinity for a wide variety of antigen peptides.

The most important self-identification molecules are called major histocompatibility complex (MHC) proteins. Each person has an almost unique combination of MHCs on the surface of their cells. The immune system of each individual normally recognizes this unique combination as self.

MHC proteins play 2 roles on the surface of cells. First, they signal to immune cells that the cell to which they are bound is self, and not a foreign cell. Second, they function as carriers to present antigens on cell surfaces.

T-Cell-Mediated Immune Responses

Once T cells leave the thymus, they are primed for generating an immune response. Occasionally, researchers may describe the specific T cell involved in tumor destruction. CD4+ and CD8+ T cells are most often described, but others exist. T cells become cytotoxic T cells as they emerge from the thymus. T cells can recognize foreign antigens through their TCRs once they come into contact with antigen-presenting cells. Then they become activated and begin fighting pathogens that bear that particular antigen.

T-Cell Recognition of Antigens

Whereas B cells directly recognize foreign pathogens, T cells need assistance from other immune system cells, antigen-presenting cells. Antigen-presenting cells include dendritic cells, macrophages, and B cells. Antigen-presenting cells ingest pathogens and break them into fragments. Antigen fragments from the invader are combined with MHC molecules as they are assembled in the antigen-presenting cell. The combination of antigen fragments and MHC molecules is moved to the cell's surface. T cells which come into contact with the antigen-presenting cell learn to recognize the invader's antigen fragments. T cells are then activated and can begin fighting the invaders with that antigen.

Suppression of Immune Responses

The suppression of the immune system happens in a few different ways, but importantly, regulatory T cells mediate suppression of immune responses carried out by other active CD4+ T cells. This process is designed to spare healthy tissue once the foreign invaders have been eradicated. This process involves functional subsets of regulatory CD4 or CD8 T cells that either secrete cytokines with immunosuppressive properties or suppress the immune response by poorly defined mechanisms that require cell-to-cell contact.

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