Lymphocyte Recognition of Pathogens: Steps from Antigen Encounter to Immune Response

Adaptive immunity relies on specific antigen recognition by B and T lymphocytes. The process unfolds in coordinated steps:

1. Antigen Recognition

• B cells: Surface immunoglobulin (BCR) binds native conformational antigen (protein, polysaccharide, lipid).
• T cells: TCR recognizes linear peptide fragment presented by MHC (Class I → CD8⁺; Class II → CD4⁺) on antigen-presenting cells (APCs: dendritic cells, macrophages, B cells).

2. Antigen Presentation and T Cell Priming

• Dendritic cells capture antigen in tissues → migrate to lymph node → present peptide-MHC + costimulatory molecules (CD80/86).
• Signal 1: TCR–pMHC binding.
• Signal 2: CD28–CD80/86 costimulation (without it → anergy).
• Signal 3: Cytokines (IL-12 → Th1; IL-4 → Th2; TGF-β+IL-6 → Th17; IL-2 → proliferation).

3. Clonal Expansion and Differentiation

• Activated T cells proliferate (IL-2 autocrine).
• CD4⁺ helper subsets: Th1 (macrophage activation, IgG), Th2 (eosinophils, IgE), Th17 (neutrophils, mucosal), Tfh (B cell help), Treg (suppression).
• CD8⁺ cytotoxic T lymphocytes (CTLs): Kill infected/tumor cells (perforin/granzyme, FasL).

4. B Cell Activation and Antibody Production

• B cell internalizes antigen via BCR → presents peptide on MHC II → receives help from cognate Tfh (CD40L–CD40, cytokines IL-4, IL-21).
• Germinal center reaction: somatic hypermutation, class-switch recombination (IgM → IgG/IgA/IgE), affinity maturation.
• Output: Plasma cells (antibody factories) + Memory B cells.

5. Effector Functions

• Antibodies: neutralization, opsonization, complement activation, ADCC.
• CTLs: direct killing.
• Macrophage activation (Th1).

6. Contraction and Memory

90–95% of effectors undergo apoptosis; long-lived memory T/B cells persist for rapid secondary response.

This choreography ensures specificity, amplification, regulation, and memory.