Antigens are molecules on pathogen surfaces that trigger immune responses

Steps:

• Recall that antigens are foreign substances recognized by the immune system.
• Identify common locations: pathogens display antigens to interact with host defenses.
• Evaluate choices: focus on primary site versus secondary or incorrect ones.
• Confirm via immunology basics: antigens primarily mark invaders for attack.

Why B is correct:

• By definition, antigens are proteins or polysaccharides on pathogen surfaces that bind to antibodies and T-cell receptors, initiating immune recognition.

Why the others are wrong:

• A: Antigens are not typically free in blood plasma; they are bound to pathogens or cells.
• C: Blood plasma contains antibodies, not antigens, which are on invading organisms.
• D: Antigens exist and are essential for immune system function.

Final answer: B