Offerings	Mar. 27, 2026 USD ($) shares
Offering: 1
Offering:
Fee Previously Paid	false
Other Rule	true
Security Type	Equity
Security Class Title	Common Stock, $0.001 par value per share, reserved for issuance pursuant to the Annexon, Inc. 2020 Equity Incentive Plan
Amount Registered \| shares	5,974,512
Proposed Maximum Offering Price per Unit	5.13
Maximum Aggregate Offering Price	$ 30,649,246.56
Fee Rate	0.01381%
Amount of Registration Fee	$ 4,232.66
Offering Note	The amount to be registered represents additional shares of common stock available for issuance under the Annexon, Inc. 2020 Equity Incentive Plan (the "2020 Plan") pursuant to the evergreen provisions of the 2020 Plan. Pursuant to Rule 416(a) under the Securities Act of 1933, as amended (the "Securities Act"), this Registration Statement shall also cover any additional shares of Registrant's common stock that become issuable under the 2020 Plan as a result of any stock dividend, stock split, recapitalization, or other similar transaction effected without the receipt of consideration that results in an increase to the number of outstanding shares of Registrant's common stock, as applicable. The proposed maximum offering price per share and proposed maximum aggregate offering price is estimated solely for the purpose of calculating the registration fee in accordance with Rule 457(c) and (h) under the Securities Act and based upon the average of the high and low prices of the Registrant's common stock as reported on the Nasdaq Global Select Market on March 24, 2026 (the "Average Price").
Offering: 2
Offering:
Fee Previously Paid	false
Other Rule	true
Security Type	Equity
Security Class Title	Common Stock, $0.001 par value per share, reserved for issuance pursuant to the Annexon, Inc. Employee Stock Purchase Plan
Amount Registered \| shares	1,493,628
Proposed Maximum Offering Price per Unit	4.37
Maximum Aggregate Offering Price	$ 6,527,154.36
Fee Rate	0.01381%
Amount of Registration Fee	$ 901.40
Offering Note	The amount to be registered represents additional shares of common stock available for issuance under the Annexon, Inc. Employee Stock Purchase Plan (the "ESPP") pursuant to the evergreen provisions of the ESPP. Pursuant to Rule 416(a) under the Securities Act, this Registration Statement shall also cover any additional shares of Registrant's common stock that become issuable under the ESPP as a result of any stock dividend, stock split, recapitalization, or other similar transaction effected without the receipt of consideration that results in an increase to the number of outstanding shares of Registrant's common stock, as applicable. The proposed maximum offering price per share and proposed maximum aggregate offering price is estimated solely for the purpose of calculating the registration fee in accordance with Rule 457(c) and (h) under the Securities Act and based upon the Average Price, multiplied by 85%, which is the percentage of the price per share applicable to purchases under the ESPP.

Offerings

Mar. 27, 2026

USD ($)

Offering:

false

true

Equity

Common Stock, $0.001 par value per share, reserved for issuance pursuant to the Annexon, Inc. 2020 Equity Incentive Plan

Amount Registered | shares

5,974,512

Proposed Maximum Offering Price per Unit

5.13

Maximum Aggregate Offering Price

$ 30,649,246.56

Fee Rate

0.01381%

Amount of Registration Fee

$ 4,232.66

Offering Note

The amount to be registered represents additional shares of common stock available for issuance under the Annexon, Inc. 2020 Equity Incentive Plan (the "2020 Plan") pursuant to the evergreen provisions of the 2020 Plan. Pursuant to Rule 416(a) under the Securities Act of 1933, as amended (the "Securities Act"), this Registration Statement shall also cover any additional shares of Registrant's common stock that become issuable under the 2020 Plan as a result of any stock dividend, stock split, recapitalization, or other similar transaction effected without the receipt of consideration that results in an increase to the number of outstanding shares of Registrant's common stock, as applicable. The proposed maximum offering price per share and proposed maximum aggregate offering price is estimated solely for the purpose of calculating the registration fee in accordance with Rule 457(c) and (h) under the Securities Act and based upon the average of the high and low prices of the Registrant's common stock as reported on the Nasdaq Global Select Market on March 24, 2026 (the "Average Price").

Offering:

false

true

Equity

Common Stock, $0.001 par value per share, reserved for issuance pursuant to the Annexon, Inc. Employee Stock Purchase Plan

Amount Registered | shares

1,493,628

Proposed Maximum Offering Price per Unit

4.37

Maximum Aggregate Offering Price

$ 6,527,154.36

Fee Rate

0.01381%

Amount of Registration Fee

$ 901.40

Offering Note

The amount to be registered represents additional shares of common stock available for issuance under the Annexon, Inc. Employee Stock Purchase Plan (the "ESPP") pursuant to the evergreen provisions of the ESPP. Pursuant to Rule 416(a) under the Securities Act, this Registration Statement shall also cover any additional shares of Registrant's common stock that become issuable under the ESPP as a result of any stock dividend, stock split, recapitalization, or other similar transaction effected without the receipt of consideration that results in an increase to the number of outstanding shares of Registrant's common stock, as applicable. The proposed maximum offering price per share and proposed maximum aggregate offering price is estimated solely for the purpose of calculating the registration fee in accordance with Rule 457(c) and (h) under the Securities Act and based upon the Average Price, multiplied by 85%, which is the percentage of the price per share applicable to purchases under the ESPP.