Question

MHC II molecules present a. processed self-antigens from proteasomes. b. processed foreign antigens from phagolysosomes. c. antibodies. d. T cell receptors.

Short answer

MHC II molecules present processed foreign antigens from phagolysosomes.

Step-by-step solution

Understanding MHC II Function

Major Histocompatibility Complex (MHC) class II molecules are found on antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells. Their primary function is to present processed antigen fragments to helper T cells (CD4+ T cells).

Identifying the Source of Antigens for MHC II

MHC II molecules typically present antigens that have been ingested by the cell and processed in phagolysosomes, which are specialized structures formed from the fusion of phagosomes with lysosomes. These antigens are generally foreign, coming from extracellular pathogens such as bacteria.

Eliminating Incorrect Options

MHC II molecules do not present self-antigens from proteasomes, antibodies, or T cell receptors. Self-antigens are usually presented by MHC class I molecules, whereas antibodies and T cell receptors have different roles in the immune response.

Selecting the Correct Answer

Since MHC II molecules present processed antigens from phagolysosomes, and these antigens are usually derived from extracellular organisms, the correct answer is that they present processed foreign antigens from phagolysosomes.

Key concepts

Antigen-Presenting Cells

Antigen-presenting cells (APCs) are crucial components of the immune system, serving as a bridge between innate and adaptive immunity. They capture pathogens, break them down into fragments known as antigens, and express these antigens on their surface using molecules called Major Histocompatibility Complex (MHC) class II.

APCs include dendritic cells, macrophages, and B cells. Dendritic cells are often the first to encounter antigens and are particularly adept at activating naive T cells. Macrophages are like sentinels in tissues, engulfing and digesting pathogens, while B cells can present antigens as part of their role in antibody production.

Function of APCs

The primary function of these cells is to process foreign antigens and present them to helper T cells. This activates the T cells, leading to a cascade of immune responses intended to combat the pathogen. Additionally, the presentation of non-foreign or 'self' antigens helps in maintaining tolerance and preventing autoimmunity.

Without effective antigen presentation by APCs, the immune system would not be able to recognize and respond efficiently to the diverse array of pathogens encountered by the body.

Helper T Cells

Helper T cells, also known as CD4+ T cells, are a subset of lymphocytes that play a significant role in establishing and maximizing the capabilities of the immune system. These cells are called 'helper' because they do not kill infected cells or fight pathogens directly. Instead, they support the activity of other immune cells by releasing cytokines, which regulate the immune response.

When a helper T cell binds to the antigen-MHC II complex on an APC, it becomes activated. Activation leads to the proliferation and differentiation of these helper T cells into various subtypes that can assist other immune cells. For instance, some helper T cells stimulate B cells to produce antibodies, while others activate cytotoxic T cells or macrophages.

Importance in Immunological Response

Helper T cells are central to the immune response, and their activation triggers a tailored response to specific pathogens. They help maintain memory cells for faster immune reactions upon subsequent exposures to the same pathogen, which is the principle behind vaccinations.

Phagolysosomes

Phagolysosomes are specialized structures within cells that contribute to the degradation of ingested material, such as pathogens. They form when a phagosome, a vesicle containing the ingested material, fuses with a lysosome, which is an organelle filled with digestive enzymes.

This fusion creates an environment where enzymes can break down complex molecules into simpler ones, including antigens that can then be presented on the cell surface through MHC II molecules. It's in these phagolysosomes that pathogens are effectively dismantled, rendering foreign antigens into forms recognizable to the immune system.

Role in Immune Defense

The formation and function of phagolysosomes are vital for the immune defense, as they limit the spread of pathogens and present their antigens for recognition by T cells. This process ensures that foreign entities can be identified and marked for destruction, while also informing the adaptive immune system to keep a memory of the pathogen for improved future responses. Thus, the phagolysosome is a critical junction between innate immunity and the start of the adaptive immune response.