Immunology Quiz- Introduction To Antigen Recognition

The Major histocompatibility complex (MHC) is a cluster of genes that encode proteins. These proteins are responsible for presenting antigens to T cells, which play a crucial role in the immune response. The MHC molecules bind to antigens and display them on the surface of cells, allowing T cells to recognize and respond to the antigens. B cell receptors (BCRs) and T cell receptors (TCRs) are not involved in encoding proteins or presenting antigens, making them incorrect choices for this description.

T cell receptors (TCRs) are only able to recognize foreign antigens if they are presented as a complex with a major histocompatibility complex (MHC) molecule. This is in contrast to B cell receptors (BCRs), which can directly recognize antigens without the need for MHC presentation. Therefore, the statement that TCRs can only recognize foreign antigen when presented with an MHC molecule is true.

The T cell receptor (TCR) is the molecule that recognizes antigens bound to the major histocompatibility complex (MHC). MHC molecules are found on the surface of cells and present antigens to T cells. The TCR specifically binds to the antigen-MHC complex, allowing T cells to recognize and respond to foreign substances. The B cell receptor (BCR) is responsible for recognizing antigens directly without the need for MHC presentation.

The major histocompatibility complex (MHC) molecules play a crucial role in the immune system by presenting peptides to T cells. This presentation allows T cells to recognize and respond to foreign antigens, initiating an immune response. By presenting peptides, MHC molecules help T cells identify infected or abnormal cells, leading to the activation of the immune system to eliminate the threat. This function of MHC molecules is essential for immune surveillance and defense against pathogens.

C3 is the correct answer because it is a key component that can be activated by all three pathways of the complement system: the lectin, classical, and alternative pathways. C3 activation is a crucial step in the complement cascade, leading to the formation of the membrane attack complex (MAC) and subsequent destruction of pathogens. Activation of C3 initiates a series of events that amplify the immune response and enhance the ability of the immune system to eliminate foreign invaders.

The B cell receptor (BCR) is a molecule that recognizes antigens and can create a soluble antigen receptor, also known as an antibody. B cells are a type of white blood cell that produces antibodies to help the immune system defend against pathogens. The BCR is located on the surface of B cells and is responsible for binding to specific antigens. When the BCR recognizes an antigen, it triggers the B cell to produce and release antibodies that can neutralize or eliminate the antigen.

Class II MHC molecules are primarily found on antigen-presenting cells such as B cells, macrophages, and dendritic cells. These cells play a crucial role in the immune response by presenting antigens to helper T cells. This interaction is important for the activation of the adaptive immune system and the production of specific immune responses against pathogens. While class II MHC molecules can also be found on other cell types, their presence on A, B, and C cells is particularly significant for antigen presentation and immune regulation.

Class I MHC molecules are found on essentially all cells. Class I MHC molecules are involved in presenting antigens to cytotoxic T cells, which play a crucial role in immune responses against intracellular pathogens such as viruses and some bacteria. Since all nucleated cells have the potential to be infected by intracellular pathogens, they express class I MHC molecules to present antigens derived from these pathogens to cytotoxic T cells. This allows the immune system to recognize and eliminate infected cells. Therefore, class I MHC molecules are found on essentially all cells.

MHCs, or major histocompatibility complexes, are extensively polymorphic. This means that they have multiple alleles or forms of the same gene. The MHC genes play a crucial role in the immune system by encoding proteins that present antigens to T cells, allowing the immune system to recognize and respond to foreign substances. The high level of polymorphism in MHC genes ensures a diverse range of antigen presentation, increasing the likelihood of an effective immune response against a wide variety of pathogens.

If the structure of an individual's MHC molecules does not allow them to recognize and bind any peptide antigen from a specific virus, it means that the individual's immune system will not be able to directly target and eliminate the virus-infected cells. However, T cells can still be activated by other means, such as through the presentation of viral antigens by other cells or through the recognition of other viral proteins. Therefore, even if the individual's MHC molecules cannot directly bind the viral antigens, they can still activate a T cell response to cells infected with that virus.

The complement system is a group of serum proteins that work together to form protein cascades, where each activated component activates the next in order to generate a physiologic response. These proteins can bind to bacteria and create holes in their membrane, which leads to bacterial lysis. They also help to eliminate immune complexes and prevent them from causing damage to the body. However, the statement that they attract phagocytes to both foreign material and self cells is not true. Phagocytes are attracted to foreign material, such as bacteria or other pathogens, but not self cells.

MHC molecules are not expressed by every cell in the body. Instead, they are only expressed on the surface of certain cells, such as antigen-presenting cells. B and T cells, on the other hand, do express a different molecule created from multiple gene segments that undergo somatic rearrangement. This process allows for the generation of a diverse repertoire of B and T cell receptors, which is important for recognizing and responding to a wide range of pathogens. The other options are incorrect because MHC molecules are indeed highly variable and polymorphic, and their diversity exists in the individual, not just the population as a whole.

BCRs (B cell receptors) can undergo hypermutation to create receptors that are an even better fit for foreign antigens. This process is known as somatic hypermutation and it occurs in the variable region of the BCR gene. Through somatic hypermutation, B cells can generate a diverse repertoire of BCRs that have a higher affinity for specific antigens, allowing for a more effective immune response. TCRs (T cell receptors) and MHCs (major histocompatibility complexes) do not undergo hypermutation in the same way as BCRs.

An individual will have about 12 different class I and II MHC molecules on the surface of certain lymphoid cells if their parents have completely different HLA genes. This is because each parent contributes half of their HLA genes to their offspring, resulting in a combination of different MHC molecules. Since there are multiple alleles for both class I and II MHC genes, the individual will have approximately 12 different combinations of these molecules.

BCRs (B cell receptors) and TCRs (T cell receptors) have an immunoglobulin fold. The immunoglobulin fold is a characteristic structural motif found in antibodies and other proteins involved in immune responses. It consists of a series of beta strands connected by loops, forming a sandwich-like structure. BCRs and TCRs play a crucial role in recognizing and binding to antigens, initiating immune responses. The presence of the immunoglobulin fold in BCRs and TCRs allows them to bind to specific antigens and activate immune cells. MHCs (major histocompatibility complexes) do not have an immunoglobulin fold, which is why they are not included in the correct answer.