The most common type of protein found in the cell membrane is:

Correct Answer: C

Rationale: The correct answer is C: Glycoprotein. Glycoproteins are the most common type of protein found in the cell membrane because they play crucial roles in cell signaling, adhesion, and recognition. They have sugar molecules attached to them, which help in cell-cell interactions. Lipoproteins (choice A) have lipids attached to them and are found in cell membranes but are not as common as glycoproteins. Mucoproteins (choice B) are proteins with carbohydrate chains but are not as prevalent in cell membranes. Nucleoproteins (choice D) are proteins associated with nucleic acids and are not typically found in the cell membrane.

Correct Answer: B

Rationale: The correct answer is B: slow-reacting substance A (leukotrienes). In severe asthma, leukotrienes are potent inflammatory mediators that cause bronchoconstriction, mucus production, and airway inflammation. Antihistamines target histamine, which is not the primary mediator in asthma. Interleukin-2 is involved in immune response modulation, not asthma symptoms. Serotonin plays a role in smooth muscle contraction, but it is not a major player in asthma pathophysiology. Bradykinin is a potent vasodilator and increases vascular permeability, but it is not the primary mediator in asthma. Therefore, the lack of relief from antihistamines suggests that leukotrienes are likely the cause of the symptoms in this patient with severe asthma.

Correct Answer: B

Rationale: The correct answer is B: immunoglobulin heavy-chain class. After binding to its specific antigen, a B lymphocyte can undergo class switching to change the type of antibody it produces while maintaining the same antigen specificity. This process involves recombination events that switch the constant region of the immunoglobulin heavy chain, leading to the production of antibodies with different effector functions. Choices A, C, and D are incorrect because they do not accurately describe the process of class switching in B lymphocytes. Light-chain isotypes, variable region of the heavy chain, and constant region of the light chain do not undergo class switching.

Correct Answer: A

Rationale: The correct answer is A: systemic anaphylaxis. When a child disturbs a wasp nest and is stung repeatedly, they can develop an allergic reaction leading to systemic anaphylaxis. This is a severe, rapid-onset allergic reaction that can cause shock due to widespread release of inflammatory mediators. Symptoms include difficulty breathing, swelling, low blood pressure, and can progress rapidly. Summary of other choices: B: Serum sickness is a delayed hypersensitivity reaction to foreign proteins, not typically seen in acute situations like this. C: An Arthus reaction is a localized immune complex-mediated hypersensitivity, not likely to result in systemic shock. D: Cytotoxic hypersensitivity involves antibodies attacking cells, not the rapid multi-system response seen in anaphylaxis.

Correct Answer: D

Rationale: The correct answer is D because toxic shock syndrome (TSS) is caused by superantigens, not A-B toxins. Superantigens directly activate a large number of T cells by binding to both the MHC class II protein and the T cell receptor simultaneously. This leads to massive cytokine release and the symptoms described. Choices A, B, and C are all accurate statements about the mechanism of action of superantigens in TSS. Choice D is incorrect because TSS toxins do not have an A-B subunit structure; they are superantigens that directly interact with T cells and do not enter cells to activate them.