A 3-year-old girl is brought to the office after developing fever and a sore throat. The patient recently entered day care, and similar symptoms have been reported in several of the other children. Physical examination shows exudative pharyngitis and enlarged anterior cervical lymph nodes. A rapid antigen detection test confirms the diagnosis of streptococcal throat infection. Her condition resolves with antibiotic therapy. Several weeks later, she is re-exposed to Streptococcus pyogenes. The bacteria penetrating beyond the surface epithelium are immediately coated with preformed IgG antibodies. Which of the following substances acts in the most similar manner to IgG antibodies to facilitate phagocytosis?

Question

A 3-year-old girl is brought to the office after developing fever and a sore throat.  The patient recently entered day care, and similar symptoms have been reported in several of the other children.  Physical examination shows exudative pharyngitis and enlarged anterior cervical lymph nodes.  A rapid antigen detection test confirms the diagnosis of streptococcal throat infection.  Her condition resolves with antibiotic therapy.  Several weeks later, she is re-exposed to Streptococcus pyogenes.  The bacteria penetrating beyond the surface epithelium are immediately coated with preformed IgG antibodies.  Which of the following substances acts in the most similar manner to IgG antibodies to facilitate phagocytosis?

Quizplus · Accepted Answer

11ec160b_37b8_c585_9249_4b8d2cad3388_MD0001_00 Opsonization occurs when host proteins such as immunoglobulins or complement bind to the surface of foreign cells (eg, bacteria) to promote phagocytosis. The most important opsonins (coating proteins) are immunoglobulin (Ig) G and complement C3b, but mannose-binding lectin and C-reactive protein can also opsonize cells. After opsonins are bound to the cell surface, they act as a handle for receptors on phagocytes (eg, Fc receptors, C3b receptors) to grasp, allowing the phagocytes to more easily engulf the foreign cell. The importance of C3 is demonstrated by the fact that all 3 complement pathways (lectin, classical, and alternative) converge on it, resulting in cleavage into C3a and C3b. C3a helps recruit phagocytic cells and induce inflammation. C3b, in addition to acting as an opsonin, can bind to C3b convertase and form C5 convertase, ultimately triggering the membrane attack complex. (Choice A) 5-Hydroxyicosatetraenoic acid (5-HETE) is produced by a variety of immune cells and serves as a leukotriene and lipoxin precursor. It also causes neutrophil and macrophage chemotaxis and neutrophil degranulation. (Choice C) Complement C5a can enhance phagocytosis by macrophages, but it does so by binding to receptors on the phagocytes and stimulating them directly, not by coating the foreign cells. It also is a potent chemotactic agent, drawing more neutrophils and monocytes to the site of inflammation. (Choice D) Although IgM can be a potent stimulator of C3b opsonization via activation of the complement cascade, there are no receptors on macrophages for the Fc portion of IgM and it does not directly enhance phagocytosis (unlike IgG). (Choice E) Leukotriene B4 is a chemotactic agent that increases the ability of leukocytes to cross from the serum into the tissues as part of an inflammatory response. (Choice F) L-selectin is an adhesion molecule that aids entry of lymphocytes from the blood into lymphoid tissue and margination of neutrophils during inflammatory responses. Educational objective: Opsonization occurs when host proteins such as immunoglobulins or complement bind to foreign cells such as bacteria and coat the surface, enhancing phagocytosis. The most important opsonins (coating proteins) are immunoglobulin G and complement C3b. __________ References: Neutrophils and macrophages work in concert as inducers and effectors of adaptive immunity against extracellular and intracellular microbial pathogens. (http://www.ncbi.nlm.nih.gov/pubmed/20110444) Complement System Part II: Role in Immunity. (http://www.ncbi.nlm.nih.gov/pubmed/26074922)

A 3-Year-Old Girl Is Brought to the Office After Developing