B Cells are activated by
a. Complement
b. Antibody
c. Interferon
d. Memory cells
e. Antigen
The correct answer and explanation is :
Correct Answer: e. Antigen
Explanation:
B cells, or B lymphocytes, are an essential component of the adaptive immune system, and their primary role is to recognize specific antigens and produce antibodies against them. The correct answer, “e. Antigen,” reflects this core function.
Activation of B Cells
B cell activation begins when a B cell encounters its specific antigen. Each B cell has a unique B cell receptor (BCR) on its surface, which is essentially a membrane-bound form of an antibody. This receptor is highly specific and can bind to a unique epitope on an antigen. When a matching antigen binds to the BCR, the B cell internalizes the antigen, processes it, and presents fragments of it on its surface via MHC class II molecules.
For full activation, especially in T cell-dependent responses, a helper T cell (CD4⁺) recognizes this antigen-MHC complex and provides additional stimulation through:
- CD40 ligand-CD40 interaction
- Cytokine release (like IL-4, IL-5, IL-21)
This additional “help” from T cells promotes:
- B cell proliferation (clonal expansion)
- Class switching (from IgM to IgG, IgA, etc.)
- Somatic hypermutation and affinity maturation
- Differentiation into plasma cells and memory B cells
In T cell-independent responses, certain antigens (like polysaccharides) can activate B cells without T cell help, usually producing a weaker, short-lived IgM response.
Why the other options are incorrect:
- a. Complement: While complement helps in immune responses, it doesn’t directly activate B cells.
- b. Antibody: Antibodies are products of B cells, not activators.
- c. Interferon: These cytokines are involved in antiviral defense but don’t directly activate B cells.
- d. Memory cells: These are the result of activation, not the cause.
Thus, antigen is the primary trigger for B cell activation.