Definition
Humoral immunity is a branch of adaptive immunity mediated by antibodies secreted by activated B cells (plasma cells).
It primarily targets extracellular pathogens, such as bacteria, viruses (before they enter cells), and toxins.
Key Players
B Lymphocytes (B Cells) – Produced in the bone marrow, these cells are responsible for antibody production.
Plasma Cells – Activated B cells that secrete large amounts of antibodies.
Memory B Cells – Persist after infection, providing long-term immunity.
Antibodies (Immunoglobulins, Ig) – Proteins that bind to antigens and neutralize them. Types include IgG, IgA, IgM, IgE, and IgD.
Mechanism of Humoral Immunity
1) Antigen Recognition
A naïve B cell recognizes an antigen via its surface B-cell receptor (BCR).
The antigen is processed and presented on MHC-II to T-helper cells (CD4⁺ cells).
2) T-Cell Help & B Cell Activation
T-helper cells release cytokines (e.g., IL-4, IL-5, IL-6) that stimulate B cell differentiation.
B cells differentiate into plasma cells and memory B cells.
3) Antibody Production
Plasma cells produce antibodies specific to the antigen.
Antibodies function by:
Neutralization – Blocking pathogen activity.
Opsonization – Marking pathogens for phagocytosis.
Complement Activation – Triggering a cascade that leads to pathogen destruction.
4) Memory Response
Memory B cells persist in the body after infection.
Upon re-exposure to the same antigen, memory B cells rapidly produce antibodies, leading to a faster and stronger immune response.
Advantages
Highly Specific – Antibodies uniquely bind to specific antigens.
Long-Term Immunity – Memory B cells ensure quicker future responses.
