Test: Immune System - MCAT MCQ

Monocytes are a type of white blood cell that circulate in the bloodstream. When they migrate from the systemic circulatory system into the general connective tissues, they differentiate into macrophages. Macrophages are highly phagocytic cells that play a key role in the immune response by engulfing and digesting pathogens, cellular debris, and other foreign substances. They are important components of the innate immune system.

Explanation: Basophils are a type of white blood cell that are involved in the allergic response and the defense against parasites. While they play a role in the immune system, basophils are not primarily responsible for phagocytosis. Instead, they release chemical mediators such as histamine and heparin to promote inflammation and allergic reactions. Neutrophils, macrophages, and eosinophils, on the other hand, are all phagocytic cells involved in engulfing and eliminating pathogens and cellular debris.

Myelogenous leukemias, also known as myeloid leukemias, are cancers that arise from the abnormal growth and proliferation of myeloid cells in the bone marrow. The bone marrow is the primary site of production of myeloid cells, including granulocytes (neutrophils, eosinophils, basophils), monocytes, and platelets. In myelogenous leukemias, there is uncontrolled growth of these myeloid cells, leading to an overabundance of immature or abnormal cells in the bone marrow and peripheral blood. The abnormal production and accumulation of myeloid cells interfere with the normal function of the immune system.

Humoral immunity refers to the component of adaptive immunity that involves the production and circulation of antibodies in the blood. Antibodies, also known as immunoglobulins, are proteins produced by B cells (a type of lymphocyte) in response to the presence of specific antigens. Once produced, antibodies are released into the bloodstream and other body fluids, where they can recognize and bind to specific antigens. This binding triggers various immune responses, including neutralization of pathogens, activation of complement system, and recruitment of other immune cells to eliminate the antigen. The circulation of antibodies in the blood allows them to reach various tissues and organs, providing a systemic defense against pathogens and foreign substances.

Recognition of self vs. non-self by the adaptive immune system is primarily achieved through the process of T cell education or maturation in the thymus. The thymus is a specialized organ located in the chest where T cells develop and differentiate. During this process, T cells are exposed to a wide range of self-antigens presented by specialized cells within the thymus called antigen-presenting cells (APCs). T cells that recognize self-antigens with high affinity are eliminated through a process called negative selection, ensuring that the mature T cells do not react against the body's own tissues. This process helps establish self-tolerance and prevents autoimmune reactions. B cells, on the other hand, mature in the bone marrow and are not directly involved in recognizing self vs. non-self during their development.

The variable portion of an antibody, which is part of the heavy chain and the light chain, is responsible for providing antigen-binding sites. The variable regions contain hypervariable regions, also known as complementarity-determining regions (CDRs), which directly interact with antigens. The CDRs exhibit high variability in their amino acid sequences, allowing antibodies to recognize and bind to a wide range of antigens. The constant portion of the antibody, on the other hand, is responsible for other functions such as effector mechanisms and determining the antibody's class or isotype.

Innate immunity is the nonspecific defense mechanism that is present at birth. It provides immediate protection against a wide range of pathogens and does not require prior exposure to the pathogen.

Macrophages are specialized immune cells that engulf and digest pathogens, cellular debris, and other foreign substances through a process called phagocytosis. They are important components of the innate immune response.

B cells are immune cells that produce antibodies. When activated by antigens, B cells differentiate into plasma cells, which secrete large amounts of antibodies specific to the encountered antigen.

Passive immunity is temporary and acquired through the transfer of pre-formed antibodies from another individual or through maternal transfer to a fetus or newborn. It provides immediate protection but does not result in long-term immunity.