Immunology

Step 1: Recall role of immune cells.
Allergic reactions involve IgE antibodies, which bind to specialized immune cells. When allergens bind to IgE, these cells release histamine, causing allergy symptoms.
Step 2: Identify the correct cell type.
Mast cells have high-affinity receptors (FcεRI) for IgE. They store histamine in granules and play a central role in allergic responses.
Step 3: Conclusion.
Thus, the correct answer is mast cells.

Step 1: Understanding antibody structure.
Antibodies (immunoglobulins) consist of constant and variable regions. The variable regions of both heavy and light chains form the antigen-binding site. These regions differ from one antibody to another and determine antigen specificity.
Step 2: Analyzing the options.
(A) Constant region: This part is responsible for effector functions, not binding.
(B) C-terminal: This is part of the constant region; not involved in antigen recognition.
(C) Variable region: Correct — antigen binding occurs here.
(D) Between constant and variable region: This area is not involved in specific antigen binding.
Step 3: Conclusion.
The antigen-binding site is entirely located in the variable regions of the heavy and light chains.

Step 1: Understand evolutionary immunology.
The immune system evolved from simple defense mechanisms in unicellular organisms to complex adaptive immunity in vertebrates.
Step 2: Trace the timeline.
- Phagocytosis originated in unicellular eukaryotes as a feeding and defense mechanism — thus predating multicellular immunity.
- Antigen presentation and thymic education are features of the adaptive immune system, which appeared later in vertebrates.
- Antibody production also evolved later as a hallmark of adaptive immunity in jawed vertebrates.
Step 3: Conclusion.
Therefore, the earliest immune-related process to evolve was phagocytosis.

Step 1: Recall the concept of plasma therapy.
In plasma therapy, antibodies from the blood plasma of recovered individuals are injected into a patient to provide immediate immunity.
Step 2: Classify the type of immunity.
This immunity is passive because the recipient receives ready-made antibodies rather than producing them through their own immune response.
It is artificial because it is induced by medical intervention, not natural exposure.
Step 3: Conclusion.
Hence, plasma therapy represents artificial passive immunity.

Step 1: Recall the structure of IgG.
An IgG molecule consists of:
- 2 heavy (H) chains, and
- 2 light (L) chains.
Step 2: Given data.
Molecular weight of light chain (L) = 24 kDa.
Molecular weight of heavy chain (H) = approximately 50 kDa (known biological constant).
Step 3: Calculate total molecular weight.
Rounded off to the nearest significant figure → 150 kDa.
Step 4: Conclusion.
Therefore, the molecular weight of human IgG is approximately 150 kDa.