Immunology #5 - T Cell

Explanation
A T cell receptor typically has one binding site. This binding site allows the T cell receptor to recognize and bind to specific antigens presented by antigen-presenting cells. The binding of the T cell receptor to the antigen triggers a series of signaling events that activate the T cell, leading to an immune response against the antigen. Therefore, the correct answer is 1.

Explanation
The alpha chain of T cell receptors contains the V (variable), J (joining), and C (constant) regions. These regions are responsible for the diversity and specificity of T cell receptors, allowing them to recognize and bind to specific antigens. The V region is responsible for antigen recognition, while the J and C regions are involved in signal transduction and cell activation. Therefore, the correct answer is V, J, C.

Explanation
RAG (recombination activating gene) is responsible for the rearrangement of genes that encode the antigen receptors on immune cells. In severe combined immunodeficiency syndrome (SCID), there is a lack of function of RAG protein, which leads to an inability to produce functional immune cells. This results in a severe impairment of the immune system, making individuals highly susceptible to infections.

Explanation
The correct answer is "Helper." CD8 is a type of T-cell that is involved in cell-mediated immunity and recognizes antigens presented on MHC class I molecules. CD8 T-cells are not considered "helper" cells, as helper T-cells are typically CD4-positive and assist in the activation of other immune cells. CD8 T-cells are primarily involved in killing infected cells and are important in the defense against viral infections.

Explanation
MHC I, or Major Histocompatibility Complex class I, is a protein that is present on the surface of almost all nucleated cells in the body. It plays a crucial role in the immune system by presenting antigens to cytotoxic T cells, which helps in identifying and eliminating infected or abnormal cells. Therefore, it is correct to say that MHC I is expressed in virtually all nucleated cells of the body.

Explanation
Class II MHC molecules are associated with dendritic cells, macrophages, and B cells. These molecules play a crucial role in the immune response by presenting antigens to helper T cells. Dendritic cells are professional antigen-presenting cells that capture and present antigens to T cells. Macrophages also perform antigen presentation and are involved in phagocytosis and immune response activation. B cells, on the other hand, are responsible for producing antibodies and presenting antigens to helper T cells. Therefore, all of these cell types are associated with Class II MHC molecules.

Explanation
Directional selection is responsible for the variations of MHC polymorphisms within different geographical areas. Directional selection occurs when a particular phenotype is favored over others, leading to a shift in the frequency of alleles in a population. In the case of MHC polymorphisms, directional selection may be driven by factors such as exposure to different pathogens or the need to adapt to specific environmental conditions. This selection pressure can result in the maintenance of multiple MHC alleles within a population, allowing for a diverse range of immune responses to combat various pathogens.