Q.91 In the human body, which of the following is/are involved in processing of a foreign
antigen?
(A) B-cells
(B) Macrophages
(C) Red blood cells
(D) Platelets

B-cells and macrophages serve as professional antigen-presenting cells that process foreign antigens in the human body. Red blood cells and platelets do not participate in this process. The correct answer is (A) and (B).​

Option Analysis

B-cells (A): B-cells function as antigen-presenting cells by binding foreign antigens via their B-cell receptors, internalizing them through receptor-mediated endocytosis, and processing them into peptides for presentation on MHC class II molecules to CD4+ T-cells. This activates helper T-cells, which in turn support B-cell differentiation into antibody-producing plasma cells.​

Macrophages (B): Macrophages phagocytose foreign antigens, degrade them in lysosomes, and load resulting peptides onto MHC class II molecules for presentation to T-cells. They bridge innate and adaptive immunity by migrating to lymph nodes to initiate T-cell responses.​

Red blood cells (C): Red blood cells lack nuclei and machinery for antigen processing or presentation; they primarily transport oxygen via hemoglobin and do not express MHC molecules.​

Platelets (D): Platelets aid coagulation and modulate inflammation but lack significant antigen-processing capability; while they express MHC class I and interact with immune cells, they do not process foreign antigens like professional APCs.​

Processing of foreign antigen in the human body is a critical immune function handled by specialized antigen-presenting cells (APCs). B-cells and macrophages excel at capturing, degrading, and presenting these antigens to T-cells via MHC molecules, triggering adaptive immunity.​

Key Cells Involved

Professional APCs dominate foreign antigen processing:

• Macrophages engulf pathogens through phagocytosis, process them in endolysosomes, and display peptides on MHC II to activate CD4+ T-cells.​

• B-cells use surface receptors for targeted uptake, process antigens efficiently, and present them to receive T-cell help for antibody production.​

Red blood cells focus on gas transport without immune roles, while platelets support clotting and minor inflammation but skip antigen processing.​

Mechanism Overview

Foreign antigens enter via endocytosis or phagocytosis, degrade into peptides, bind MHC II, and surface-present for T-cell recognition. This pathway excludes non-APCs like RBCs and platelets. For CSIR NET aspirants, note: only B-cells and macrophages qualify here.​