Which antibody isotype is most abundant in circulation?

Correct Answer: B

Rationale: The correct answer is B: IgG. IgG is the most abundant antibody isotype in circulation, constituting about 75% of total antibodies. This is because IgG is produced during secondary immune responses and has a longer half-life compared to other isotypes, allowing it to accumulate in circulation. IgA is mainly found in mucosal surfaces, IgE is associated with allergic reactions, and IgM is the first antibody produced in a primary immune response. Therefore, IgG is the most abundant in circulation due to its longevity and role in secondary immune responses.

Correct Answer: B

Rationale: The lag phase of the primary antibody response refers to the time it takes for the immune system to generate specific antibodies after initial exposure to an antigen. The correct answer is B (5-10 days) because during this period, B cells are activated, undergo proliferation, differentiate into plasma cells, and start producing antibodies. This process takes time as the immune system needs to recognize the antigen, mount a response, and produce sufficient antibodies. Option A (1-3 days) is too short for the full activation and differentiation of B cells. Option C (10-15 days) is too long for the typical lag phase duration. Option D (No lag phase) is incorrect because there is always a lag phase before the peak antibody production in the primary immune response.

Correct Answer: B

Rationale: The correct answer is B: IgE. In Type I hypersensitivity reactions, IgE antibodies are involved. Firstly, upon initial exposure to an allergen, B cells produce IgE antibodies specific to that allergen. IgE antibodies then bind to mast cells and basophils. Upon subsequent exposure to the same allergen, it cross-links the IgE antibodies on mast cells and basophils, leading to degranulation and release of inflammatory mediators like histamine. This results in the symptoms of Type I hypersensitivity reactions. Choice A (IgA) is incorrect because IgA is mainly involved in mucosal immunity. Choice C (IgG) is incorrect because while IgG can be involved in Type II and Type III hypersensitivity reactions, it is not the primary antibody in Type I hypersensitivity. Choice D (IgM) is incorrect because IgM is typically involved in the primary immune response, not in Type I hypersensitivity reactions.

Correct Answer: B

Rationale: The correct answer is B because Type IV hypersensitivity reactions are delayed and T cell-mediated. Upon exposure to an antigen, sensitized T cells, specifically CD4+ T cells, release cytokines that recruit and activate macrophages and other effector cells, leading to tissue damage. Choice A is incorrect because Type I hypersensitivity reactions are immediate and IgE-mediated. Choice C is incorrect because immune complex-mediated hypersensitivity reactions are typically associated with Type III hypersensitivity. Choice D is incorrect because histamine release by mast cells is characteristic of Type I hypersensitivity reactions, not Type IV.

Correct Answer: D

Rationale: The correct answer is D: Systemic lupus erythematosus (SLE). Type I hypersensitivity involves IgE-mediated reactions to allergens, leading to immediate responses like asthma, food allergies, and anaphylaxis. SLE is an autoimmune disease involving immune complexes and not IgE antibodies. Therefore, SLE is not directly associated with Type I hypersensitivity. Asthma, food allergies, and anaphylaxis are all examples of Type I hypersensitivity reactions due to IgE-mediated mechanisms.