1. Antigen-Presenting Cells (APCs):
* Macrophages: These cells engulf pathogens and display fragments of their antigens on their surface using MHC (Major Histocompatibility Complex) molecules.
* Dendritic cells: These cells are highly efficient at capturing antigens and presenting them to T cells, initiating the immune response.
* B cells: Though primarily involved in antibody production, B cells can also present antigens to T cells, particularly during the initial stages of an immune response.
2. T lymphocytes (T cells):
* Helper T cells (Th cells): These cells recognize antigen-MHC complexes on APCs and activate other immune cells. They release cytokines that stimulate B cell proliferation and differentiation, and promote cytotoxic T cell activation.
* Cytotoxic T cells (Tc cells): These cells directly kill infected cells by releasing cytotoxic substances. They recognize antigen-MHC complexes on infected cells.
* Memory T cells: These cells persist after an infection and can quickly recognize and respond to the same pathogen if encountered again, providing long-lasting immunity.
3. B lymphocytes (B cells):
* Plasma cells: These cells are the antibody factories of the immune system. They are differentiated from B cells and produce large quantities of antibodies specific to a particular antigen.
* Memory B cells: These cells remain after an infection and can quickly differentiate into plasma cells upon re-exposure to the same antigen, providing a rapid and robust antibody response.
4. Other cells involved in active immunity:
* Natural killer (NK) cells: These cells are part of the innate immune system but can also contribute to active immunity by killing infected cells and producing cytokines.
* Mast cells: These cells are involved in allergic reactions and release histamine, which can contribute to inflammation and the recruitment of other immune cells.
The overall process of active immunity involves:
1. Antigen presentation by APCs
2. Activation of T cells
3. Proliferation and differentiation of B cells into antibody-producing plasma cells
4. Antibody production and elimination of the pathogen
5. Generation of memory cells for long-lasting immunity
This is a simplified overview, and the process is much more complex and involves intricate interactions between various cells and signaling molecules.
