Question

Which of the following are the main mediators/ initiators of type II hypersensitivity reactions? a. antibodies b. mast cells c. erythrocytes d. histamines

Short answer

The main mediators/initiators of type II hypersensitivity reactions are antibodies (option a).

Step-by-step solution

Understanding Type II Hypersensitivity Reactions

Type II hypersensitivity reactions, also known as cytotoxic reactions, occur when the immune system mistakenly targets and damages its own cellular components. Typically, these reactions are mediated by antibodies, specifically IgG and IgM, that are directed against surface antigens on cells or extracellular material.

Identifying the Mediators

The main mediators of type II hypersensitivity reactions are antibodies that bind to antigens on the surface of cells such as erythrocytes. This binding can lead to cell lysis or phagocytosis. While mast cells and histamines play a role in other types of hypersensitivity reactions, they are not the initiators of type II hypersensitivity.

Eliminating Incorrect Options

Mast cells (option b) and histamines (option d) are more closely associated with type I hypersensitivity reactions. Erythrocytes (option c) are potential targets of type II hypersensitivity reactions but are not the initiators. Therefore, the main mediators/initiators are antibodies (option a).

Key concepts

Understanding Hypersensitivity Reactions

Hypersensitivity reactions are an overreaction of the immune system to various substances, which can range from benign to potentially harmful. These reactions are categorized into four types: I, II, III, and IV, based on their immune mechanism and timeline.

In these conditions, the immune system, which usually defends the body against infections and disease, mistakenly identifies a harmless substance as a threat. This misidentification leads to an immune response that can cause tissue damage, inflammation, and a variety of symptoms.

Type II Hypersensitivity Reactions

Specifically, Type II hypersensitivity, or antibody-mediated hypersensitivity, involves the direct binding of antibodies, such as IgG and IgM, to the antigens present on the surface of cells. This can lead to cellular damage through different mechanisms, including complement activation and phagocytosis.

• Common conditions associated with this reaction include hemolytic anemia, Goodpasture's syndrome, and rheumatic fever.
• This reaction can be initiated by certain drugs, infections, or underlying autoimmune disorders.

Role of Immune System Mediators

Immune system mediators are molecules that convey signals and orchestrate the complex interactions between cells of the immune system and other bodily tissues. These mediators include a wide array of substances such as cytokines, chemokines, antibodies, and cell surface receptors.

Antibodies as Mediators

Antibodies play a crucial role across various immune responses. They are produced by B cells and have the specific task of identifying and neutralizing foreign objects like bacteria and viruses.

• Antibodies can also be involved in hypersensitivity reactions, where they may mistakenly target self-antigens on normal cells.
• Mediators like mast cells and histamines are prominent in Type I hypersensitivity reactions, known as immediate hypersensitivity, which is the mechanism behind allergies and anaphylaxis.

Understanding how different mediators work is critical for diagnosing and treating hypersensitivity reactions.

Antibody-Mediated Cytotoxicity

Antibody-mediated cytotoxicity is a specific immune response where antibodies directed against self antigens on cell surfaces lead to their destruction. This is the principal mechanism behind Type II hypersensitivity reactions.

When antibodies bind to these self antigens, they can trigger a range of destruction pathways:

• They can activate the complement system, leading to the formation of a membrane attack complex and cell lysis.
• They can facilitate phagocytosis, where immune cells engulf and digest the marked cells.
• Antibody-dependent cellular cytotoxicity (ADCC) can also occur, where natural killer cells recognize the antibody-coated cells and kill them.

This mechanism plays a pivotal role in autoimmune diseases, where the body's immune system attacks its own tissues. Understanding this process is essential for developing targeted therapies that can minimize tissue damage while moderating the immune response.