Adhesion molecules and co-stimulatory molecules on the APC membrane reversibly bind to the corresponding ligand molecules on the initial T cells to mediate adhesion and prepare for the next step of specific binding.

TCR recognizes pMHC, conducts signals through CD3, and enhances the expression abundance and affinity of adhesion molecules on the membrane.

CD4/CD8 recognize and bind to APC/target cell surface MHC-II/MHC-class I molecules respectively

Recognition and binding of TCR-CD3 complex and antigen peptide/MHC complex

CD4/CD8 bind to MHC-Ⅱ/MHC-Ⅰ class molecules respectively (MHC-restricted)

Many co-stimulatory molecules on APC bind to corresponding co-receptors on T cells

Cytokines promote full activation of T cells

1. Mediates cellular immunity; 2. Secretes cytokines to activate, induce and recruit macrophages; 3. Produces IL-2 and other cytokines to promote the activation and proliferation of Th1, Th2, CTL and other cells; 4. Secretes IFN-γ to promote the production of B cells with opsonization The acting Ab further enhances the phagocytosis of pathogens by macrophages; 5 can activate neutrophils and prompt them to kill pathogens

1. Assists humoral immune response; 2. Participates in hypersensitivity inflammation, participates in hypersensitivity reactions and anti-parasitic infection; 3. Secretes cytokines to participate in B cell activation and differentiation, regulates the type conversion of immunoglobulins, and produces different types of Ab.

1. Secrete cytokines to participate in inflammatory reactions, autoimmune diseases and infectious diseases; 2. Chemotaxis stimulates neutrophils and monocytes; 3. Enhances the inflammatory response of vascular endothelial cells and induces local inflammatory responses.

FasL/Fas pathway

perforin/granzyme pathway

B cells provide BCR specific recognition of antigens, and Igα/Igβ transmit signals

B cell co-receptor CD19/CD21/CD81 signals

CD40 on the surface of B cells binds to CD40L on the surface of activated Th cells

Create B cells with various affinities, leading to BCR diversity and diversity of Ig production

The body's immune system gradually produces high-affinity antibodies

High-frequency mutations in somatic cells → create B cells with different affinity BCRs

Follicular dendritic cells → Screen B cells with high affinity BCR

The antigen recognition specificity of the antibody remains unchanged, but the type of antibody changes

The antigen concentration is high, the incubation period is long, and the antibodies are mainly IgM.

The antigen concentration is low, the incubation period is short, and the antibodies are mainly IgG.